Tribune was a title shared by elected officials in the Roman Republic. Tribunes had the power to convene the Plebeian Council and to act as its president, which also gave them the right to propose legislation before it. They were sacrosanct, in the sense that any assault on their person was prohibited. They had the power to veto actions taken by magistrates, and specifically to intervene legally on behalf of plebeians. The tribune could also summon the Senate and lay proposals before it. The tribune's power, however, was only in effect while he was within Rome. His ability to veto did not affect regional governors.
Because it was legally impossible for a patrician to be a tribune of the plebeians, the first Roman emperor, Augustus, was offered instead all of the powers of the tribunate without actually holding the office (tribunicia potestas). This formed one of the two main constitutional bases of Augustus' authority (the other was imperium proconsulare maius). It gave him the authority to convene the Senate. Also, he was sacrosanct, had the authority to veto (ius intercessionis), and could exercise capital punishment in the course of the performance of his duties.
Most emperors' reigns were dated by their assumption of tribunicia potestas, though some emperors, such as Tiberius, Titus, Trajan and Marcus Aurelius had already received it during their predecessor's reign. Marcus Agrippa and Drusus II, though never emperors, also received tribunicia potestas.
By extension from the Roman historical experience, some modern politicians have been called "Tribunes of the People." The analogy is mostly with the original function of the Tribunes as champions of the downtrodden and disadvantaged in Roman society, rather than with the later technical Roman governmental usage giving legitimacy to Imperial power.