What is the "First Communion" and what is the significance?

It's when a person has been acknowledged as a full member of a church, and takes part in the sacrament of Holy Communion for the first time. Each denomination is different, I believe most people have to take a class learning about the church's doctrines and pass a test. The Lutheran church I was forced to attend as a child allowed it for kids when they were 13, I think Catholics do it younger.