The central part of Christian life in antiquity and the Middle Ages was the Mass - this was when you went to church and the priest went through a series of prayers to God. You would participate in some of the prayers, sometimes just saying Amen, and sometimes saying other things - these were called "responses". Both you and the priest said the Mass in Latin (in Europe and most of North Africa) or in Greek (in Greece and Egypt and West Asia), which meant that a lot of people in medieval Europe didn't understand the words very well, because they spoke French or Spanish or Italian or German and not Latin.
Here's a video of a priest saying Mass in England
At the end of the prayers, the priest gave Communion to everyone who was not in a state of sin. That meant that you had not done anything wrong, or if you had, you had confessed it to the priest and he had forgiven you.