Medieval architects designed most Romanesque and Gothic churches starting with the design of a Roman basilica. But many churches added a part coming out crossways. That made the whole church take the shape of a cross. It gave you more room for people to see the mass. And it reminded people how Jesus died on the cross. This sideways part was the transept.
Generally architects designed the church with the front, or façade (where people came in) to the west. The altar (where the priest stood to say Mass) was to the east, facing Jerusalem where Jesus was crucified.
People called the east part of the church the choir. That was behind the altar, where the choir sang. The apse (pronounced APPS) was the rounded end of the choir. They called the west end of the church the nave. People stood in the nave to watch the priest say Mass. The aisles were the side parts off the nave. In the Middle Ages, churches usually didn’t have chairs in them, and people stood up, or they brought their own folding chairs.