Learn Hardcore Catalan: Quan el meu germà truca al timbre, la meva mare baixa al portal. - When my brother rings the doorbell, my mother goes down to the building entrance.

This is especially normal with family members and many everyday nouns. So Catalan usually says the my brother / the my mother in a way that sounds natural in Catalan, even though English does not do that.

If you leave out the article, it can sound literary, old-fashioned, or stylistically marked in many contexts.

What does quan mean here exactly?

Quan means when, but in this sentence it often has the sense of whenever.

Because both verbs are in the present tense, the sentence usually describes something habitual or repeated:

Whenever my brother rings the doorbell, my mother goes down to the entrance.

So quan can introduce:

a specific time: when
a repeated situation: whenever

Here, the repeated meaning is the most likely one.

Why are both verbs in the present tense: truca and baixa?