What is thread basically? ?

Answer 1

A thread is a particular flow of control within a process. A thread shares the address space with other threads in the same process, but each thread gets its own call stack and set of registers. Switching between threads within a process is a much lighter-weight operation than switching between processes.

Conceptually, there are two major types of thread: cooperatively scheduled threads and preemptively scheduled threads. With cooperatively scheduled threads, it is up to the thread to manually yield control to the task scheduler (though this usually also happens on I/O) and with preemptively scheduled threads, every thread may be interrupted and suspended by the scheduler at any point. The advantage of cooperatively scheduled threads is that it is easy to make sure that control is only transferred to other threads when the state of the system is logically consistent with higher-level constraints, but preemptively scheduled threads can make much better use of additional hardware.

Answer 2

This link will give some more information about the mention
https://en.wikipedia.org/wiki/Thread_(computing)

Answer 3

Like a mini process or program that is part of a larger program/app/application.