The Plebeian Tribunes (not to be confused with military tribunes) were the only people who represented the plebeians. The tribunes were able to protect the interests of the plebeians mainly by vetoing actions that had been taken by the various magistrates. These included financial decisions that were made by the quaestors, sentences handed down by praetors, and laws made by the consuls. They were supposed to use these powers to protect the plebeians from unjust actions by the patrician magistrates.
The power of the tribunes did not extend to actually making positive laws. Instead, it was a purely negative power that was used to prevent injustice.