iPhone 5s - What is the dot between rear camera and the LED flash?
It's a microphone, useful for video recording using the rear camera and for digitally eliminating background noise when you're on a phone or FaceTime call. Source - Apple support.

Apple doesn't have an explicitly stated opinion on whether or not the rear microphone absolutely can't be covered, but in that support article,


  Remove screen protectors, films, or cases that cover the microphone, iSight mic, or iPhone receiver.


is the third recommended step for people experiencing microphone issues.