IFF is a radar-based system to help in determining whether a detected aircraft is friendly or an enemy.

Excerpt from Wikipedia:

Identification, friend or foe (IFF) is an identification system designed for command and control. It enables military and civilian air traffic control interrogation systems to identify aircraft, vehicles or forces as friendly and to determine their bearing and range from the interrogator. IFF may be used by both military and civilian aircraft. IFF was first developed during the Second World War, with the arrival of radar, and several infamous friendly fire incidents.

Despite the name, IFF can only positively identify friendly targets, not hostile ones.^[1][2][3][4] If an IFF interrogation receives no reply or an invalid reply, the object cannot be identified as friendly, but is not positively identified as foe (it may, for instance, be a friendly aircraft with an inoperative or malfunctioning transponder). There are in addition many reasons that friendly aircraft may not properly reply to IFF.

IFF is a tool within the broader military action of Combat Identification (CID), “the process of attaining an accurate characterization of detected objects in the operational environment sufficient to support an engagement decision.” The broadest characterization is that of friend, enemy, neutral, or unknown. CID not only can reduce friendly fire incidents, but also contributes to overall tactical decision-making.^[5]