Je vais prendre ceux-ci, parce qu’ils sont en coton et moins chers.

Ceux-ci means these ones or these.

It is a demonstrative pronoun, which means it replaces a noun that is already understood from the context. For example, in a shop, if you are looking at several shirts, shoes, or other items, you do not need to repeat the noun.

So instead of saying:

Je vais prendre ces pantalons-ci

you can simply say:

Je vais prendre ceux-ci

This is used for a masculine plural noun. If the hidden noun were feminine plural, you would use celles-ci instead.

Both refer to those/these ones, but they differ in distance or emphasis:

• ceux-ci = these ones / the ones here
• ceux-là = those ones / the ones there

In practice:

• -ci often points to something closer
• -là often points to something farther away or contrasts with another group

Example:

• Je préfère ceux-ci, pas ceux-là.
  = I prefer these ones, not those ones.

In everyday French, the distinction is sometimes less strict than in English, but this is the basic idea.

The hyphen is standard when -ci or -là is added to a demonstrative pronoun or demonstrative adjective.

So you write:

• ceux-ci
• ceux-là
• ces livres-ci
• cette robe-là

The hyphen links the pointing word -ci or -là to what it modifies.

Je vais prendre is the near future: literally I am going to take.

French often uses aller + infinitive to talk about a decision or intention, especially in everyday speech. In a shopping situation, it sounds very natural.

• Je vais prendre ceux-ci. = I’m going to take these.
• Je prendrai ceux-ci. = I will take these.

Both can be correct, but je vais prendre usually sounds a bit more immediate and conversational.

Because que becomes qu’ before a vowel sound. This is called elision.

So:

• que ils → qu’ils
• que elle → qu’elle
• que on → qu’on

French avoids the clash of two vowel sounds in this kind of structure.

Ils refers back to ceux-ci.

Since ceux-ci is plural and masculine, the sentence continues with ils sont:

• ceux-ci → ils
• celles-ci → elles

So the grammar matches the implied noun that ceux-ci stands for.

Because chers is an adjective, and it agrees with the noun it describes.

Here, it describes the items referred to by ceux-ci, which is masculine plural, so the adjective must also be masculine plural:

• masculine singular: cher
• feminine singular: chère
• masculine plural: chers
• feminine plural: chères

So:

• ceux-ci sont moins chers
• celles-ci sont moins chères

In French, en is very commonly used to indicate the material something is made from, especially with clothing and manufactured items.

So:

• en coton = made of cotton
• en laine = made of wool
• en cuir = made of leather

This is just the normal French pattern in many cases. English often uses of, but French frequently prefers en.

Because the comparison is left implicit.

Moins chers means less expensive / cheaper, but the sentence does not say exactly than what. The listener is expected to understand it from the context, for example than the others.

If you want to state the second part explicitly, you add que:

• Ils sont moins chers que les autres.
  = They are cheaper than the others.

Without que, it simply means they are cheaper / less expensive in a general contextual sense.

Yes, car can also mean because, but parce que is the most common and natural choice in everyday spoken French.

• parce que = everyday, very common
• car = more formal, more common in writing or careful speech

So in a normal shop conversation:

Je vais prendre ceux-ci, parce qu’ils sont en coton et moins chers.

sounds very natural.

You would need feminine forms:

• Je vais prendre celles-ci, parce qu’elles sont en coton et moins chères.

Notice all the changes:

• ceux-ci → celles-ci
• ils → elles
• chers → chères

This is because French keeps agreement with the gender and number of the noun being replaced.

It is pronounced roughly like suh-see, but with French vowel sounds:

• ceux sounds like seu
• ci sounds like see

Important points:

• the x in ceux is silent
• the c in ci sounds like s

So the whole thing is approximately:

ceux-ci → seu-see

Not perfect English spelling, but that is a useful guide for beginners.