A Wireless Access Point, or W-A-P, WAP, or simply AP, is a key component in a WiFi network. The Access Point is the device that WiFi clients connect to.
In a way, it’s similar to a switch, however wireless clients do not need physical ports to connect to.
However, there usually will be a port or interface so the AP can connect to a switch. This makes it possible for wired and wireless devices to be on the same network.
You probably have a wireless router at home. This router connects you to the internet, probably through a cable or phone line. This router probably also has a Wireless Access Point built in, so your phones and tablets can have internet access.
In a larger network, for example, a warehouse or an office building, there are probably many Access Points. This improves wireless coverage.
Coverage is the area that the radio signal can reach. The more coverage, the larger the area where you can connect to WiFi.