At one point he served as the Patriarch of Venice. Then in 1973, Paul VI made him a Cardinal. Once he was elected Pope, he served for only 33 days, before passing away.
John Paul I chose that name as a way to honor his two predecessors: John XXIII and Paul VI. He was actually the first pope with two names.
The theme of his papacy was 'Humility'. It's a virtue he characterized from the time of his coronation ceremony, when he rejected the traditional papal tiara.