What was it? The First Ecumenical (all the churches) Council, convened by many Bishops at the Request of Constantine, the newly Christianized emperor, in 325. It dealt with the problem of Arius, who had been condemned by local councils, and Arianism. Arianism (not to be confused with its homonym Aryanism, a racist position) is the belief that Christ was not equal with God, and instead was his first Creation. Thus he was near God, but not on the same level of God. This was rejected in favor of a Christology that saw God as the same as Father and Son who were “co-eternal.” Thus the Son was not “begotten” in time, but “eternally begotten of the Father” meaning the “begotten-ness” is the characteristic of their relationship, not a temporal event.
Who was involved? Between 300-400 Bishops attended, together with the priests they brought with them, some counts exceed 1200. Although many important people were there, probably the most influential was Athanasius.
Where might I have heard of it? Pop culture, especially pseudo-historical fiction such as Dan Brown, likes to mention it, since it was the first council, and claim it said odd things. In fact it said little about the Trinity, nothing about the canon of Scripture, and nothing about the role of the Roman Emperor.
Fun Fact: It also established the dating of Resurrection Sunday, relative as it is now to the Jewish lunar calendar.
Why it was important? Well it was the first council, which makes it pretty important in that it was a cooperative effort. Essentially, it formally acknowledged what Christianity was going to be. The Nicaean/Arian controversy would dominate the early church for decades, also establishing the agenda for other important historical moves. Also, it recognized that in Jesus, God himself came to meet us, and did not send a proxy. Thus Jesus is rightly praised and we are rightly humbled by his action. It also gave us the Nicene Creed, which many churches recite from time to time.
How’d I do? Any topics you’d like to see compressed down? Longer discussion allowed in the comment