What Is a Thread?
We write code in a file line by line, and then it gets executed. To be able to execute a piece of code requires an execution environment. In Java, a thread is an execution environment. If a program has only one execution environment, then we call this program a single-threaded program.
Interestingly, in Java, we can create a lot of threads to run different parts of the code of a program executing independently. And when we have achieved that, we call it a multi-threaded program.