When two squares in the same row, column or box can each only be the same two numbers, say 3 and 7, then those two squares will take the 3 and the 7 between them. You don't know which is which yet, but you do know that no other square in that unit can be a 3 or a 7.
Naked pairs don't place a number right away. They clean up your pencil marks, and that cleanup usually reveals a single somewhere else. They are the gateway to Medium puzzles.