1) In wireless network technology, to transmit voice and data traffic from a cell site to a switch, i.e., from a remote site to a central site.
(2) In satellite technology, to transmit data to a point from which it can be uplinked to a satellite.
In example – if you have a sattelite downlink for data but have a dialup to upload data, then the dialup link is the backhaul. Some of the original ‘cable’ modems were backhaul devices – downloading over the cable, but sending requests and uploads over a dialup connection.
In the question the described wireless system uses backhaul for connecting the various APs. Your connection is direct to and from the AP, but the cable between origin (switch) and various APs is backhaul.