Learn Portuguese (Portugal): Ela compra uma saia.

You would replace the word uma (a) with a demonstrative word. Because saia (skirt) is feminine, you use esta (this). The sentence becomes: Ela compra esta saia. If you wanted to say "that skirt", you would use essa or aquela.

Why is the verb compra and not comprar?

Comprar is the dictionary form of the verb, meaning "to buy". Because we are talking about what "she" does, we have to change (conjugate) the verb to match. For regular "-ar" verbs, we drop the "-ar" and add "-a" for the "he/she/it" form, giving us compra.

Why do we use uma instead of um?

In Portuguese, words for "a" or "an" must match the gender of the noun they describe. Saia is a feminine noun, so we use the feminine word uma. If she were buying a masculine item, like a book (livro), we would use um: Ela compra um livro.

Does uma mean "a" or "one"?

It can mean both! The words um and uma serve as both the indefinite article ("a" or "an") and the number one. You can usually tell from the context whether the speaker simply means "a skirt" or is emphasizing that she is buying exactly "one skirt".