Deadlock is a condition that arises when a set or group of processes are in blocked state, because each process has a resource and is waiting for other resource, that is already acquired by any other process.
Due to this situation, no process gets the desired resources. This is just similar to a one way track, with just a capacity of one big vehicle at a time, as when two big vehicles are in opposite direction, they will block each other's way and on one will be able to move ahead. None of them can either vacate the road.
This example better explains the meaning of deadlock condition, that when a requested resource is already occupied by other process, none of them will pre-empt until they get an access to the desired resource, which will continue them blocked.