Well, you could always resort to reading the Bible, I guess, and you’ll find it here. The footnote says it’s Aramaic for “Father”, but idiomatically I believe it’s more like “Daddy” - a child’s word.
There were some who claimed that one of Jesus’ innovations was referring to God so informally as “Abba”. But it turns out not to be true. There are records of other street preachers, Jesus’ near-conteporaries (like Honi the Circle-Drawer), using the same term.
