A basilica is a fancy building that people use as a church. It's usually really big and has a special shape - kind of like a rectangle with a big, long hallway down the middle. This hallway is called the nave. Along the sides of the nave, there are usually a bunch of little rooms called chapels where people can pray or do other religious stuff.
At the front of the basilica, there's a big area called the sanctuary. This is where the altar is, and where the priest or other religious leader stands when they're doing a special ceremony. Sometimes there's also a place in the sanctuary called the apse - it's like a little room that sticks out from the rest of the building, and it usually has a big, fancy decoration like a picture or a statue.
People have been building basilicas for a really long time - like, thousands of years! There are lots of different kinds of basilicas, depending on what religion people use them for and where they are in the world. But they're all similar in that they're big, fancy buildings with special places for people to pray and worship.