Na tej stronie książki jest ciekawy opis.

Because the preposition na requires the locative case when it means location, and both ta and strona have to change form.

• ta → tej
• strona → stronie

So:

• ta strona = this page (dictionary/basic form)
• na tej stronie = on this page

This is very common in Polish: after a preposition, the noun often changes case, and any adjective or demonstrative word changes with it.

Here książki is in the genitive singular because it means of the book.

So the phrase:

• strona książki = page of the book

This is a very common Polish pattern:

• okładka książki = the cover of the book
• autor książki = the author of the book
• tytuł książki = the title of the book

So in the full sentence:

• na tej stronie książki = on this page of the book

It is not locative here, because książki does not depend directly on na. It depends on stronie.

Because na only affects stronie, not książki.

The structure is:

• na tej stronie = on this page
• książki = of the book

So the sentence is built like this:

• on this page
  • of the book

If you said na książce, that would mean on the book physically, as if something were lying on top of the book.

Compare:

• na stronie książki = on a page of the book
• na książce = on the book

So książki stays genitive because it expresses possession/relationship: the page belonging to the book.

In this sentence, jest means there is / is and sounds natural and standard.

• Na tej stronie książki jest ciekawy opis. = There is an interesting description on this page of the book.

Polish often omits jest in some kinds of sentences, especially when saying that one thing equals another:

• To mój brat. = This is my brother.

But in an existential sentence like there is/there are, jest is normally used:

• W pokoju jest stół. = There is a table in the room.
• Na tej stronie książki jest ciekawy opis.

So here, omitting jest would sound incomplete or unnatural.

Because ciekawy opis is the grammatical subject of the sentence, not a direct object.

The structure is like:

• On this page of the book
  • there is
    • an interesting description

In Polish existential sentences with jest, the thing that exists is usually in the nominative:

• Jest problem. = There is a problem.
• Na stole jest książka. = There is a book on the table.
• Na tej stronie książki jest ciekawy opis.

So:

• opis is nominative singular
• ciekawy matches it in gender, number, and case

Because opis is a masculine noun, so the adjective must also be masculine.

• opis = masculine
• ciekawy = masculine singular nominative

Polish adjectives agree with the noun in:

• gender
• number
• case

Examples:

• ciekawy opis = interesting description
• ciekawa książka = interesting book
• ciekawe zdjęcie = interesting photo

So ciekawa opis would be incorrect because the adjective form does not match the noun.

Opis usually means description, account, or depiction.

In this sentence, it most naturally means description.

Depending on context, opis can refer to:

• a written description in a book
• a description of a place, person, event, or object
• a summary-like descriptive passage

So ciekawy opis suggests that the page contains some descriptive text that is interesting.

By itself, yes: strona can mean:

• page
• side
• website/webpage

But here książki makes the meaning clear:

• strona książki = page of a book

So in this sentence, strona definitely means page, not website.

No, Polish word order is flexible, but the given order is natural and neutral.

• Na tej stronie książki jest ciekawy opis.

This puts the location first and then introduces what is there.

You could also say:

• Ciekawy opis jest na tej stronie książki.

That is grammatical, but it gives more emphasis to ciekawy opis.

The original version is more natural if you are simply stating what appears on that page.

Polish has no articles, so it does not directly mark a/an/the the way English does.

That means:

• opis could mean a description or the description
• książki could mean of the book or of a book

Context decides.

In this sentence, English would most naturally translate it as:

• On this page of the book, there is an interesting description.

The word tej already gives some definiteness to page because it means this:

• na tej stronie = on this page

So even without articles, the sentence is perfectly clear in Polish.

Grammatically, that is close, but in natural English on this page of the book sounds much better.

Polish often uses a structure like:

• strona książki = page of the book

English sometimes prefers:

• this page of the book

rather than:

• this book page

So the most natural English version is still:

• On this page of the book, there is an interesting description.

Yes. Znajduje się is another common way to say is located / can be found.

So:

• Na tej stronie książki jest ciekawy opis.
• Na tej stronie książki znajduje się ciekawy opis.

Both are correct.

The version with jest is simpler and more everyday.
The version with znajduje się sounds a bit more formal or descriptive.

A useful way to see the structure is this:

• Na tej stronie książki = a location phrase
• jest = the verb
• ciekawy opis = the thing that exists in that location

So the sentence breaks down as:

• Where? → na tej stronie książki
• What is there? → ciekawy opis

That is why opis is nominative: it is the main noun of the clause.
And książki is genitive because it only modifies stronie.