People come up to me as I leave the stage after a performance and tell me tey saw my mother onstage with me every time I sing. I keep a sense of humor about it.

This quote means that after the speaker’s musical performances, people often tell him that his mother was onstage performing with him. He does not deny this and instead chooses to keep a sense of humor about it. This means that he still laughs at the situation, although it may not be true that his mother was there singing with him.