1.e4 e5 2.Qh5 is frightening the first time you meet it, because the queen is suddenly staring at f7 and it feels like you have already lost. You have not. You have been handed free moves, and the way to use them is to attack the queen every time she settles.
A tempo is one move treated as time. Play 2…Nc6, then 3.Bc4 g6, and look at what has happened to the clock nobody is watching.
8 
7 
5 
2 
a 
b c 
e g h 1 Counting the free moves
By move five White has played the queen twice and will play her again. Black has put a knight on c6, kicked the queen with g6, brought a knight to f6 (which blocks the queen’s route to f7, so Scholar’s Mate is dead) and fianchettoed a bishop on g7. Four moves, four pieces improved. White has one bishop out and a queen doing laps.
That difference is the whole concept. Black gained tempi not by playing fast but by playing moves that were useful and forced an answer at the same time. Every check, every attack on a loose piece, every pawn push that hits something buys you a move if the reply does nothing for the other side.
The three ways club players give them away
Moving the same piece twice in the opening for no reason is the first, and the queen is the usual culprit. The second is the automatic edge pawn: h6 and a6 played out of habit, one move each, neither of them developing anything. The third is a retreat you could have avoided by looking one move ahead, which is a whole move handed back for nothing.
None of these is a blunder. Nobody hangs a piece. That is exactly why they go unnoticed for years, and why an engine calls them inaccuracies rather than mistakes.
Buying time on purpose
Once you can count tempi you can spend them. Handing over a pawn to gain three or four moves is a gambit, and it is a sensible trade because a pawn buys about three tempi in the opening. In an endgame the currency gets even harder: with only kings and pawns left, a single spare move decides who has to give way first, and whole endings are won by losing a move on purpose.
If you want to feel the difference between a move that gains time and one that just passes it, opening puzzles are the fastest way to see it on a real board.