If you want to take a shower, and there's someone already using the bathroom, you'll have to wait. How does a thread handle this?
It's done with something called mutual exclusion. It means pretty much what you think — a number of threads are mutually exclusive when it comes to a particular resource.
If you're taking a shower, you want to have exclusive access to the bathroom. To do this, you would typically go into the bathroom and lock the door from the inside. Anyone else trying to use the bathroom would get stopped by the lock. When you're done, you'd unlock the door, allowing someone else access.
This is just what a thread does. A thread uses an object called a mutex (an acronym for MUTual EXclusion). This object is like the lock on a door — once a thread has the mutex locked, no other thread can get the mutex, until the owning thread releases (unlocks) it. Just like the door lock, threads waiting to obtain the mutex will be barred.
Another interesting parallel that occurs with mutexes and door locks is that the mutex is really an advisory lock. If a thread doesn't obey the convention of using the mutex, then the protection is useless. In our house analogy, this would be like someone breaking into the washroom through one of the walls ignoring the convention of the door and lock.