Why ask for consent?
Asking consent means asking for permission. For example, if you want to touch someone, have sex with someone or kiss. Also during sex, it is important to check if your sex partner still wants to. And after sex, check whether they still feel good about it. That way, you know that the sex was a good experience for the other person.
Consent is something you do together. So it's not only important that you ask consent from your sex partners, but also that you yourself can indicate what you do and do not want.
Read how to give and withdraw consent here.