A Basic Service Set (BSS) is the basic building block of a Wi-Fi network. It is the group of stations that can talk to each other at Layer 2 on the same channel. In normal infrastructure Wi-Fi a BSS consists of one access point and the clients that are associated to it. That BSS is identified by a BSSID, which is the MAC address of the access point radio. The network name you see, the SSID, can be the same across many BSSs, so large sites create several BSSs on different access points and let devices roam between them. Those related BSSs form an ESS, which is a larger network tied together by the wired back end known as the distribution system. An access point can host more than one BSS at the same time by advertising multiple SSIDs, each with its own BSSID and often its own VLAN. The coverage area of one BSS is sometimes called the basic service area. Management frames such as beacons define the BSS on air and carry details like supported rates, security, and timing information for power save. In 802.11ax you will also see a BSS colour, a small number in the beacons and headers that helps devices tell overlapping BSSs apart on the same channel so they can reuse airtime more efficiently. There is also an independent BSS, better known as an ad hoc network, where devices talk directly without an access point and use a locally created BSSID.
