A stereotype is a preconceived notion, especially about a group of people. Many stereotypes are racist, sexist, or homophobic.
Have you ever heard someone say Irish people are all drunks, or African-American people are always late, or women are bad drivers? Those are stereotypes: commonly held ideas about specific groups. You most often hear about negative stereotypes, but some are positive. For example, there's a stereotype that Asian people do better in school. One of many problems with any stereotype is that even if it's true in some cases, it's certainly not true in all cases.