Insiders are the people who are in the know when it comes to a company and its secrets. Insiders in a company include its officers and directors, large stockholders of the company, and anyone else who would be in possession of important information that the average person doesn't have. Insiders also include immediate family members of all the above.

The important thing about insiders is that they have access to insider information, and if you somehow overhear or learn of this information, you have to be really careful not to use it in trading or you could end up in a legal mess—or in jail.

Insiders themselves have to obey a bunch of rules to make sure they don't use the knowledge they have to gain a trading advantage. 

Find other enlightening terms in Shmoop Finance Genius Bar(f)