A mistress is someone who has a special romantic relationship with a person who is already married or in a committed relationship with someone else. Sort of like sharing a toy, when you are supposed to be playing with another toy. It’s not very nice or fair to the other person who is supposed to be playing with the toy, right?
Some grown-ups might have special connections with other people who are not their "official" boyfriend, girlfriend or spouse. This kind of connection is a secret, and they keep it hidden from their families or partners. Having a mistress is not a good or honest thing to do, as it can cause a lot of hurt and is not respectful of the person they are supposed to be committed to. Instead, people should work on building honesty, trust and strong relationships, and not cheat or hurt the people they love.