Sharing limited resources
You can think of a single-threaded program as one lonely entity moving around through your problem space and doing one thing at a time. Because theres only one entity, you never have to think about the problem of two entities trying to use the same resource at the same time, problems like two people trying to park in the same space, walk through a door at the same time, or even talk at the same time.
With multithreading, things arent lonely anymore, but you now have the possibility of two or more threads trying to use the same limited resource at once. Colliding over a resource must be prevented, or else youll have two threads trying to access the same bank account at the same time, print to the same printer, adjust the same valve, and so on.