Guest Network
From DD-WRT Wiki
A virtual Guest Network is a separate SSID (wireless network ID) for guests that gives them access to the WAN (Internet) while blocking them from your LAN (local network) thereby protecting your security.
Another common use of a Guest Network is to provide Internet access for an adjacent rental unit or a neighbor.
Prior to 23020
23020 and later
Kong added easy Guest Network capability to DD-WRT starting with build 23020.
Instructions for how to set it up: Guest Wifi Setup DD-WRT
Important Note
Even though it's isolated from your LAN, your Guest Network should have a good network password to prevent unauthorized {ab)use of your Internet service, and it should be a different password from your wireless LAN.