In simple terms, it's like a hub for wireless NICs to connect to.
You know how you normally have hubs or switches with cables for each computer? Well, a wireless access point acts as a wireless hub through which you access the network.
Don't confuse access points and routers, though. You'll see many WAPs with integrated routers, usually for sharing a broadband internet connection. If you're looking at building a wireless network, only get the router if you have need for it. If you already have a router, get an access point.
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