What is an Active Electronically Scanned Array or AESA?

Active Electronically Scanned Array or AESA is a phased array system in which the beam of signals can be steered electronically in any direction, without physically moving the antenna. The antenna consists of an array of an array of small antennas each with a separate feed. The beam is steered by electronically by controlling the phase of the radio waves transmitted and received by each of the multiple radiating elements in the antenna. This digitally controlled scanning nature of the AESA allows it to quickly scan any direction in comparison to a mechanically scanned radar, whose range is constrained by the direction it is facing and how quickly its motors can turn it.

AESAs are mainly used in radars providing incomparable detection, targeting, tracking, and self-protection capabilities. In AESA, every element has its own transmit/receive module (TRM), which makes this system more reliable as the failure of a single element won't hinder the functioning of the system.
This is an image of an AESA from MACOM. Each Antenna element has a seperate feed, amplifier and heat sink.

Two main applications of Active Electronically Scanned Arrays are 5G and radars 

5G uses beam steering technology that operates using AESA technology to increase throughput and deliver the high data rates required.

Radars that use AESA technology do not need to be mechanically steered and have less moving parts which increase the lifetime of the radars and lowers the maintenance costs. In addition to the physical benefits, AESA based radars are less susceptible to Electronic Jamming. Traditional radars use the same frequency to broadcast the signals, which the jammers determine and transmits a signal at that same frequency to confuse the radar receiver. But the AESA radars use a technique called “chirping”, by which they transmit the signals over a wide band of spectrum making it difficult for the jammers to jam an AESA system. AESA radars also have low interception, as they don’t have a fixed pulse repetition frequency, which reduces the antenna radar signature and hence makes it less traceable. So they are often used to build radars on airforce planes.