Parallel is an API which allows you to multitask.
Functions are not actually executed simultaniously, but rather this API will automatically switch between them whenever they yield (eg whenever they call coroutine.yield(), or functions that call that - eg os.pullEvent() - or functions that call that, etc - basically, anything that causes the function to "pause").
Each function executed in "parallel" gets its own copy of the event queue, and so "event consuming" functions (again, mostly anything that causes the script to pause - eg sleep(), rednet.receive(), most of the turtle API, etc) can safely be used in one without affecting the event queue accessed by the other.
|parallel.waitForAny(function function1, function function2, ...)||number stoppedFunction||Runs all the functions at the same time, and stops when any of them returns.|
|parallel.waitForAll(function function1, function function2, ...)||nil||Runs all the functions at the same time, and stops when all of them have returned.|