AP is ACCESS POINT, access point means; with this access point, the computer can access the Internet, the usual use of the router is a kind of AP.

Gain antenna is a better understanding of it, this antenna is similar to the PA, can increase the signal strength. For example, an antenna with a gain of B, the transmitter transmits a signal that reaches the antenna port with a power of A. After the antenna, the signal strength is A + B.

The antenna can be used to transmit a signal to a computer.