An access point (AP) connects to the network by cable and broadcasts a Wi-Fi signal around it. One AP covers a limited area and a limited number of devices, so a large office or several floors need multiple access points.
Unlike a home router, business APs are mounted on ceilings or walls, are often powered via PoE, and are managed centrally — so all points work as one network.
Example
A two-floor office has 2 APs on each floor — the signal is strong in every room.