Okay, so sometimes in certain cultures and religions, men are allowed to have more than one wife. This is called polygamy.
But sometimes, a man may want to have another woman in his life without getting married to her. She may not be his wife, but he wants to have a relationship with her. This is where the idea of a pilegesh comes in.
Basically, a pilegesh is like a sort of girlfriend or a mistress. She is a woman who is in a relationship with a man, but they are not legally married. This is usually allowed in certain religious communities, but not all.
It's important to remember that while it may be allowed in some cultures, relationships should always be respectful and consensual. It's never okay to use someone or treat them badly just because they are not your wife.