Spanning Tree Protocol
From DD-WRT Wiki
The Spanning Tree Protocol (STP) is a protocol that tries to find the best route to use when multiple routes are available to the same destination. In these cases, mesh networks without STP enabled begin to experience Loopback. The need for STP occurs when your router is being used in a mesh network with multiple WDS-enabled repeaters, or in an adhoc network with multiple adhoc connections.
If your router is not being used in either of the given scenarios, then likely STP is not necessary. Therefore, it is recommended that you disable it.
Users of Comcast Cable should always disable this option and leave it disabled, as STP conflicts with the routers DHCP client for the WAN connection. (Comcast Cable Problem Only)