A cell site is a big machine that helps your cell phone talk to other people's cell phones. Think of it like a big speaker that sends out a signal so that your phone can hear it and send signals back.
The cell site is like a really tall tower that has antennas on it that look like long sticks with balls on top. The antennas send and receive signals from your phone, and they're usually put up really high so they can reach as far as possible.
When you're talking on your phone, your voice is turned into special signals that the cell site can understand. The cell site then sends those signals to the person on the other end of the line by turning them back into sound waves.
Cell sites are super important because without them, your phone wouldn't be able to make and receive calls and texts. They're all over the place, and you'll often see them on top of tall buildings, water towers, or even disguised as trees!