Root Guard is a feature of STP (Spanning Tree Protocol) that puts an interface into a blocked state. The interface cannot become a path to the root switch, nor can it reside on the root switch. Root Guard is useful when a Layer 2 network includes connections to switches that it does not control -- for example, a customer network and its connections to a service provider network. The customer would want to enable Root Guard on interfaces that connect to the service provider network.
You can enable Root Guard selectively on switch interfaces by choosing Device > STP and using the Root Guard tab on the STP window.