If you want to heat water until it boils, and you watch it while you wait, then it seems to take a very long time. In the same way, anything that we wait for with eager attention seems to take a very long time: like waiting for someone to arrive, waiting for the phone to ring, waiting for a letter to come.
We all have a place in this world and we all perform a function, regardless of our ability or disability. The word order of this sentence may make it more difficult to understand. In normal English it would be something like: "They (those people) who only stand and wait, also serve."