A misdemeanor is a minor offense, rather than a serious crime. A minor infraction like keeping a library book for years or stealing a pack of bubblegum would be considered a misdemeanor.
A crime like murder is serious, a felony that can land the person who commits it in jail for a long time. A misdemeanor, on the other hand, is a far less serious offense. Commit a misdemeanor such as petty theft, and you might not get more than a slap on the wrist.