A futex is an ephemeral list of waiting threads or processes keyed on an address. It can be used to implement any synchronization or signaling primitive, like mutexes, condition variables, semaphores, etc.
This is a really nice insight into the kernel's model of what's going on. It's probably not the best way for an absolute beginner to think about why they want this, but it's much better than some of the other explanations if you have trickier technical or performance questions, and it makes it obvious why it's clever and why you need the OS kernel to actually implement it.
