The Wireless Filter menu allows you to specify the MAC addresses of the only devices that will be allowed to connect on wireless interfaces. It is effectively a MAC Address whitelist for wireless access.

MAC Address: Enter the 12-digit MAC address of the client device you wish to allow to access the wireless network.

Description: This optional, but highly recommended field allows you to enter a plain text description of the device. This is helpful for keeping track of which device is which.

Finally, click Save for the changes to take effect.

Clicking Add will create a new row in the table in which you can add another MAC address.

You can choose how and on which interface this list will work on the Advanced/Virtual Wireless (Interfaces) menu. There is a link in the Notes section of the Wireless Filter page to take you there.