Daar is 'n verskil tussen bysiendheid en versiendheid.

Here Daar is means there is. It is the normal Afrikaans way to introduce the existence of something, just like English there is / there are.

So:

Daar is 'n verskil ... = There is a difference ...

A useful extra point: Afrikaans also uses is with plural nouns in this pattern:

Daar is twee verskille = There are two differences

Not in this sentence. Daar can mean a real location, but it can also be a kind of dummy subject in expressions like Daar is ...

Compare:

Die boek is daar. = The book is there.
Here daar is a location.

Daar is 'n probleem. = There is a problem.
Here daar does not mean a place; it just introduces the sentence.

In your sentence, it is the second use.

'n is the Afrikaans indefinite article, the equivalent of English a / an.

So:

'n verskil = a difference

It is pronounced very weakly, like uh or a schwa sound, never as a full stressed word. Also, unlike English, Afrikaans does not switch between a and an. It is always just 'n:

'n boek = a book
'n appel = an apple

This is just how standard Afrikaans spells the indefinite article. The apostrophe is part of the word.

A very important writing rule is that 'n stays lowercase even at the beginning of a sentence. Instead, the next word gets the capital letter:

'n Verskil bestaan.

Not:

'N verskil bestaan.

This is a detail many learners notice early, because it looks unusual from an English point of view.

Because 'n means a/an, while een means one.

So in your sentence:

Daar is 'n verskil ... = There is a difference ...

If you said:

Daar is een verskil ...

that would sound more like There is one difference ..., with emphasis on the number one.

tussen ... en ... means between ... and ...

So:

tussen bysiendheid en versiendheid = between nearsightedness and farsightedness

This is the normal pattern in Afrikaans:

tussen X en Y

You need both parts, just as in English you normally say between X and Y.

The suffix -heid is very similar to English -ness. It turns an adjective or descriptive form into an abstract noun.

For example:

moontlik = possible
moontlikheid = possibility

In your sentence, -heid helps form nouns meaning the condition or quality of being something:

bysiendheid = nearsightedness
versiendheid = farsightedness

Yes. That is often a very helpful way to learn Afrikaans vocabulary, because Afrikaans forms many compounds quite transparently.

You can think of them roughly like this:

bysiendheid

• by = near / close
• siend = seeing
• -heid = -ness

versiendheid

• ver = far
• siend = seeing
• -heid = -ness

So they are built in a very logical way, roughly near-seeing-ness and far-seeing-ness.

The sentence follows a very common Afrikaans pattern:

Daar + is + noun phrase + extra information

So:

Daar + is + 'n verskil + tussen bysiendheid en versiendheid

A useful grammar point is that Afrikaans main clauses usually place the finite verb early in the sentence. In this kind of existential construction, daar comes first and is follows immediately after it.

That is why the sentence is:

Daar is 'n verskil ...

and not something like:

Daar 'n verskil is ...

which would be wrong in a normal main clause.