Military tribunes (tribuni militum) were officers of the Roman Legion and a step towards entry into the senate for young men of social rank.
In the Republican period, there were six appointed to each legion and various duties of command and administrative were rotated between them. After the reforms of Marius (c. 107 BC) the tribunes were appointed to specific duties.
The second in command to the legatus was the tribunus laticlavius. This was a position of future advancement for a young (early 20s) senator or senatorial candidate and would provide invaluable experience for later proconsular or propraetorian commands.
The other five tribunes tribuni angusticlavii were slightly lower in rank and likely of equestrian social rank (Related forum thread: Equestrians And Patricians), but were more likely to be professional officers of experience.
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Tribunes were often political appointments. Young senators working their way up the ladder and receiving favors as part of the Roman client system.