An informant is someone who secretly provides information to police, the FBI, or another law enforcement agency. If you watch a lot of crime shows on TV, you're probably familiar with informants.
In the world of law enforcement, informants are officially called CIs, for "confidential informant" or "criminal informant." The information informants provide can be useful in catching criminals, and the informant usually gets something in return — money or a reduced prison sentence, for example. Sometimes informant simply means "person who gives information."