So, a wireless access point is what you would use to give wireless access to a device, that doesn't have wireless access. Think about it as a converter between regular LAN, which is just ethernet cables, to a wireless world.
For example, if I were to have a printer that has a network connection, or a computer that has a network connection, but doesn't have any wireless capabilities, I can then connect the printer or the computer to the wireless access point. And once the wireless access point is configured, the wireless access point can connect for the computer or printer, to my wireless network. That's the basic thing that wireless access points do.
They could also be used to do other things, like expand a wireless network or, if I have a wireless router somewhere that is not large enough, I can attach my access point to the wireless router with a cable, and then have more wireless access in a different part of the house.
Now, almost every router out there can also act as an access point, if you configure it like that. But access points on their own, should be a little cheaper and a little simpler to operate. And that's the basics of what a wireless access point is.