Kin,
This command is used to tell the switch whether or not frames arriving on the interface carry CoS, IP Precedence, or DSCP values that can be trusted. For instance, if a port on your 3550 is connected to an upstream switch that is marking all traffic as it enters the network, you can configure the port on the 3550 to trust those markings as they enter the switch (3550). On the other hand, ports that are configured as untrusted cause the 3550 to re-write the CoS value to zero.
-Zach