A thread is one path of work inside a running program. A program can have more than one thread, letting it handle several activities without starting a completely separate program for each one.
Imagine two workers sharing a desk and the same set of notes. One can respond to a screen while another handles a background task.
Threads share parts of the program's memory, which can make it easier for them to share information. It also means they need care when changing the same information at the same time.