What is meant by thread in java?

I'd ask you to read more about Concurrency. Threads isn't only in java but it's a way of managing or controlling how a program gets processed and how the OS and the CPU treats the program.

The main goal of threads is executing small parts of the process of one program in parallel. Thanks to multi-core CPUs, we can divide a program or a process into small parts, called threads, and send each thread to one core for processing. Certain requirements should be met for the threads to get executed at the same time but that's the main point of threads and multi threading.