Programs with multiple threads can have problems, like the threads of a story in which if proper synchronization does not occur, then things go wrong. What if our soldier went into battle before the battle or war even existed? Weird.
What if we have a variable, int x, that represents a key piece of data that three threads of our program use. What happens if one thread gets slightly ahead of itself and makes the data "wrong" for the other two? This problem is the problem of correctness caused by multiple threads racing to completion obliviously—because after all, they are just dumb code.
The problem of correctness can be solved by close oversight of the threads and locking, locking meaning temporarily preventing execution in one thread to be sure things are working in a synchronized manner. This is kind of like preventing a soldier from boarding a ship to war until the ship has actually docked and the gang plank has been lowered, avoiding an embarrassing splash.
The other...