Setting up a MAC Filter

Step 5 What the MAC filter does...

Every wireless network adapter has a MAC Address imprinted after it is produced. The router reads the MAC address when a wireless device tries to access it. If the MAC address doesn't match what's on the list, no connection is possible.

This security isn't perfect. MAC Addresses are burned into the wireless client's hardware, but some allow for soft-changes to impersonate other addresses. I've seen websites that claim MAC filters are easy to get around with the right equipment and know-how. I've never had a problem, but if security is really important to you, you can combine this with other security measures.

The benefit of NOT adding other security is that your wireless connection appears as unsecured, which is nice if you want to boot a leech in a clandestine manner. As long as the leech isn't familiar with MAC address hacking, you'll probably be fine.

For more info check out Wikipedia, it has a good article on MAC Addresses. Also check this out to see the weaknesses of MAC filters.