בחורף יש פחות אנשים בחוף, ולכן הוא שקט יותר.

בחורף means in winter / in the winter.

The prefix ב־ means in / at / during, and חורף means winter.
So בחורף is a time expression telling you when the rest of the sentence is true.

Hebrew often puts a time expression near the beginning of the sentence, just like English can do:

• In winter, there are fewer people at the beach.
• בחורף, יש פחות אנשים בחוף.

In unpointed Hebrew text, בחורף can cover both ideas, and in many everyday contexts the difference is not very important.

A useful way to think about it is:

• in winter = general seasonal meaning
• in the winter = a slightly more definite English phrasing

Hebrew often does not force the same distinction that English does here. So in a sentence like this, בחורף is best understood naturally as in winter.

Because יש is the standard Hebrew word for there is / there are.

So:

• יש אנשים = there are people
• יש פחות אנשים = there are fewer people

Hebrew does not use a present-tense form of to be the same way English does in existential sentences.
So יש is exactly what you expect here.

Yes. In modern Hebrew, פחות is commonly used for both:

• less
• fewer

So פחות אנשים means fewer people.

Unlike English, Hebrew does not usually make a strict everyday distinction between less and fewer.
Also, פחות does not change form here to match the noun.

Examples:

• פחות זמן = less time
• פחות אנשים = fewer people
• פחות מכוניות = fewer cars

The singular is usually איש for man/person in this kind of pattern, and its plural is the irregular form אנשים.

So:

• איש = man / person
• אנשים = people

This is a very common word, and the plural is not formed in the regular way, so it is worth memorizing as a set.

Here בחוף is best understood as at the beach or on the beach.

The prefix ב־ often covers several English prepositions depending on context, including:

• in
• at
• sometimes on

With locations, Hebrew often uses ב־ where English would choose different words depending on the situation. So בחוף is a natural way to say at/on the beach.

In unpointed Hebrew writing, בחוף can represent either idea, and context usually tells you how specific it is.

So it may be understood as:

• at the beach
• on the beach
• sometimes more generally at a beach

In this sentence, English will usually translate it naturally as at the beach.

ולכן means and therefore, therefore, or so.

It is made of:

• ו־ = and
• לכן = therefore / so

In this sentence it connects the cause and the result:

• fewer people at the beach
• therefore it is quieter

ולכן is normal and common. It can sound a little more formal or written than a very casual אז in some contexts, but it is not strange or overly formal.

הוא refers to החוף — the beach.

So the logic is:

• There are fewer people at the beach in winter,
• and therefore it is quieter.

Hebrew uses הוא because חוף is a masculine singular noun.

Because חוף is masculine.

Hebrew adjectives and pronouns have to agree with the noun they describe. Since חוף is masculine singular, Hebrew uses:

• הוא = he/it
• שקט = quiet, masculine singular

If the noun were feminine, you would expect feminine forms. For example:

• העיר שקטה יותר = the city is quieter

Here, because חוף is masculine:

• החוף ... הוא שקט יותר

Hebrew usually forms the comparative with:

• adjective + יותר = more + adjective

So:

• שקט = quiet
• שקט יותר = quieter / more quiet

This is much simpler than English, because Hebrew does not usually change the adjective itself the way English does with quiet → quieter.

More examples:

• גדול יותר = bigger
• מהיר יותר = faster
• יפה יותר = more beautiful / prettier

Yes. Hebrew word order is fairly flexible, though some versions sound more natural than others.

The original sentence is natural and clear:

• בחורף יש פחות אנשים בחוף, ולכן הוא שקט יותר.

But you could also say something like:

• בחורף החוף שקט יותר, כי יש בו פחות אנשים.
  In winter the beach is quieter, because there are fewer people there.

The original version first states the reason, then the result. That is why ולכן fits so well.