Who was Barabbas?:
Barabbas was a criminal whom the gospels say was released by the Romans instead of Jesus. Bar-abbas means son of the father, thus a false son of the father was chosen by the Jews to be released while the true son of the father was delivered to the Romans for crucifixion. Some early manuscripts of Matthew give Barabbas a surname: Jesus. This may have been a copyist error because the Greek words for to you (release to you Barabbas) could have looked like an abbreviation for Jesus.
When did Barabbas live?:
Assuming that Barabbas even existed, which is unlikely, we have absolutely no information about when he was born or when he died.
What did Barabbas do?:
The gospel authors describe Barabbas variously as a bandit, a thief, and a participant in political insurrection. Romans typically treated political rebels as bandits, crucifying them along a busy road as a warning to others.
Why was Barabbas important?:
There was no custom whereby the Romans would release a condemned prisoner on the occasion of a holy day, but even if there were Pilate would never consent to allow Barabbas go over Jesus. No one who tried to overthrow Roman rule was allowed to live. Jesus, even if he did claim to be King of the Jews, hadnt done anything overt or violent yet.
Why did Mark make up Barabbas and the custom of letting prisoners go on a holy day? Probably for the same reason that he made up the image of crowds of Jews calling out for Jesus to be crucified: it allows him to shift the blame for Jesus death away from the Roman authorities and onto the shoulders of the Jews. Marks community of Jewish Christians lived outside Palestine and under Roman rule. They would have been watched and had the authorities found them developing a religious sect after a man executed by Romans for being a political revolutionary, they would have cracked down even harder.