Hi Pep,
Welcome to the forum.

Chaste is an old word and not very popular in current English. Having said that, you might still see it knocking around and so I'll try to explain its meaning.
It does mean something like 'faithful,' but is more connected with the idea of sexual intercourse. It is an adjective which refers to people who don't have sex (are celibate), usually because they aren't married and to do so would be a sin. It's exactly because of modern attitudes about sex that we don't use this word very much these days.
Hope that helps.