Explanation

Root guard does not allow the port to become a STP root port, so the port is always STP-designated. If a better BPDU arrives on this port, root guard does not take the BPDU into account and elect a new STP root. Instead, root guard puts the port into the root-inconsistent STP state which is equal to a listening state. No traffic is forwarded across this port.
Reference: http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10588-74.html