Entrapment is the action of tricking someone into committing a crime by means of fraud or undue influence (=where a person takes advantage of their position of power over another person) so that they can then be prosecuted. Specifically, entrapment requires a law enforcement official, typically a police officer, to defraud or unduly pressure the suspect (=a person that a person investigating or prosecuting a crime thinks has probably committed that crime) to take the steps necessary to commit a crime.
From a public policy perspective, setting traps to catch criminals is considered acceptable because it is in the public interest to catch criminals. Further, there is nothing inherently wrong with setting traps for those who would otherwise commit crimes. The problem arises when the traps are likely to cause persons who are not otherwise predisposed to criminal activity to commit crimes.
Entrapment is also the name of the defence of having been entrapped. To establish this defence, the defendant must prove that he would not have committed the crime if not for the illegal actions of the law enforcement official.