AP == Access Point, in the context of Wifi discussions.
Basically, what people think of as a "Wifi router", is actually, an Access Point (or several, if it's multi-band), a router (with a WAN and LAN ports), and a switch (with all of the LAN ports connected).
An Access Point, bridges between ethernet and wifi, essentially. A standalone one, generally does not have DHCP, WAN routing, or NAT features. You can turn an ordinary wifi router, into an AP, by disabling those features in the firmware settings, generally-speaking.
I believe that most Asus router's running their own AsusWRT firmware, offer the option between Router and AP, as well as Media Bridge and Wifi Client mode, depending on model.