Watch a queen chase a king around an empty board with no help. She can give check every single move and get nowhere.
h 1 Every check is real, and not one of them threatens mate. The queen tours d8, e8 and f8 while the king ambles from d4 to g4, no worse off than when he set out. The reason is arithmetic. Mate needs the king attacked and every square beside him covered, and from a distance the queen always leaves him at least one. Step in right next door and she does cover them, but nothing is defending her, so he takes her and the game is drawn on the spot.
Your king turns her into a mating piece
Bring the white king up and everything changes, because the two of them divide the work: he guards her, she does the checking, and the escape squares are covered twice over. That is king and queen against king, and it takes ten moves at most from anywhere on the board.
The only way to spoil it is stalemate, the classic version being a black king on h8 with the white queen dropping in on g6, which is a real risk precisely because the queen covers so much. If the lone king has no legal move and is not in check, the game is drawn and your extra queen is worth nothing.
When the defender has boxed himself in
None of the above applies when the enemy king still has his own pieces around him, because then they take away his escape squares for you. This is the ordinary back rank mate and the queen does it with the white king sitting in the corner.
7 a d That is worth internalising as the difference between an endgame and a middlegame. In the endgame, with a bare king, the queen needs a partner. In a middlegame, where the defender’s own army is in the way, one queen on the right square finishes the game by herself.
The same is true of a single rook, a single bishop and even a single pawn: a mate is delivered by one piece, but the cage is built by several, quite often by the loser. If you want to see it happen a hundred times in an evening, back rank mate puzzles are the fastest way, and how to stop blundering in chess covers the other half of the lesson, which is not walking into one yourself.