Often the queen can attack a loose piece while at the same time not checking the enemy king but threatening mate. The enemy has to address the mating threat, so the loose piece is lost just as it would be if the queen had delivered a check. To master this pattern it is important to understand a couple of principles about mating threats and how to spot them. It usually takes two pieces to create a mating threat (or mate itself, of course). The threat of a back rank mate is a prominent exception, but for now just think about a classic set of cases that does follow the pattern: your queen is poised to attack a square next to the enemy king that already is attacked by one of your other pieces. At the same time your queen attacks a loose piece. Your opponent has to defuse the mate threat just as he would a check, giving you a move to take down the target.
The diagram to the left illustrates the idea in skeletal form. White has a bishop trained on g7. His queen can threaten mate by jumping to d4 or g4 because either move creates the possibility of Qxg7#. Those mate threats will make a fine anchor for a queen fork of any loose pieces Black leaves within range of d4 or g4. The White X’s in the diagram thus indicate squares that White indirectly controls because of the possible fork. (He already controls d7 with his queen; but imagine a pawn on d2 and the control again becomes indirect.) If Black leaves a loose piece on any of them (or even a piece defended once but also attacked once by another White piece), White may be able to win it by playing one of his two mate threats, Qd4 or Qg4. Black will have to respond to the mate threat by stepping forward one of the pawns in front of his king or in some other way, and then White will win the target. In addition to taking Black pieces left unguarded on those squares, White also can carry out other operations there (exchanges, or putting his own pieces onto the squares) with more confidence than might appear to the untrained eye.
In real play the details of what White can do here naturally will depend on which piece Black has left on those vulnerable squares, and of course on other pieces on the board that will complicate the picture. The e6 square is defended by a pawn; White wouldn’t be able to use his queen to attack a bishop on a diagonal; etc. Or suppose Black has a loose bishop on b4 and White forks it with Qd4 or Qg4. Black can save his piece by playing the bishop back to f8; this both removes it from danger and adds a defender to g7, thus defusing the mate threat. But these are details. The important thing for now is just to realize that in this humble-looking formation White has a strong forking threat waiting to be unleashed, that it isn't related to any check he can give, and that it gives him a measure of control over many squares that appear not to be under attack.