Most commonly a WiFi radio will operate in one of three modes: Master, Managed, Monitor. Additional modes include ad-hoc, mesh and repeater and are both less common and outside the scope of this guide.
An Access Point (or simply AP) will operate in Master Mode while client devices operate in Managed Mode. Monitor mode, sometimes called RFMON for Radio Frequency MONitor, is a special mode that allows the radio to passively monitor all traffic in the given area.
Keep in mind that not all radios have each of these capabilities and a radio can only operate in one mode at a time.