Learn Dutch: Wij kopen meestal vetarm vlees. - We usually buy low-fat meat.

The suffix -arm literally means poor, so vetarm translates to poor in fat. In English, we usually say low-fat. It is a highly productive suffix, meaning you can attach it to many nouns to indicate a low amount of something, such as suikerarm (low-sugar) or zoutarm (low-sodium).

Why is there no -e at the end of vetarm here?

Because vlees is a het-word (neuter noun) and it is being used indefinitely in this sentence. When an adjective comes before an indefinite het-word, it does not get the standard -e ending. If you had said the low-fat meat, it would be het vetarme vlees.

Could I use vetvrij instead of vetarm in this sentence?

Grammatically yes, but the meaning changes. The suffix -vrij means completely free of something. Therefore, vetvrij vlees means fat-free meat, while vetarm vlees means low-fat meat.

Why is meestal placed right after the verb kopen?

In a standard Dutch main clause, adverbs of time or frequency like meestal typically sit directly after the conjugated verb. You could also start the sentence with it for emphasis, which would invert the subject and verb to keep the verb in the second position: Meestal kopen wij vetarm vlees.