Sharing a sense of humor means both people find the same things funny. A person's "sense of humor" describes the things that they find funny. So, if two people have the "same sense of humor" or "share a sense of humor", they laugh at the same types of things. If one person has one type of "humor" and the other one does not, they do not share a sense of humor, and it means they do not find the same things funny.