Bluetooth is a way for devices such as computers, phones, and speakers to talk to each other. Just like you and your friends need to talk in order to play a game together, devices need to talk to each other in order to share information. Bluetooth is like a special language that only these devices understand.
When two Bluetooth devices are near each other, they use special radio waves to talk to each other. These radio waves create a connection between the two devices, kind of like a magical invisible bridge. The devices can now send information back and forth over this invisible bridge, just like friends can talk to each other over the phone.
The cool part is that you don't have to plug in any wires or setup anything special to get them to talk to each other. Once the connection between the two devices is established, they can talk without you having to do anything special.