A stereotype is a preconceived notion, especially about a group of people. Many stereotypes are rooted in prejudice — so you should be wary of them.
You have probably heard stereotypes: commonly held ideas or preconceptions about specific groups. You most often hear about negative stereotypes, but some are positive — the stereotype that tall people are good at basketball, for example. 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.