The German word zwischenzug means 'in-between move', and it is a common tactic that occurs in almost every game of chess!
Picture your opponent making a move that directly threatens one of your pieces: instead of responding directly to the threat, you make another even more forcing move, like a check or mate threat.
When this tactic is used in the game of chess you will make a move that poses an even more devastating threat, instead of countering a direct threat as the opponent expected you to. Often the move that you made will be a direct attack against your opponent's queen or the king. Your opponent will be forced to counter that threat against his or her queen or king first, and this will ideally change the situation to your disadvantage.
As easily as you can use this tactic, so can your opponents. Because this is a common, well-known tactic you should always watch out for a zwischenzug. Do not assume that the opponent has to counter your threats immediately, no matter how great they may appear in your eyes. It is good practice to always check whether your opponent has a check or a move that can threaten your queen. Conversely, anticipate your opponent's threats and plan a surprising Zwischenzug.
Try to use this tactic when it appears to work to your advantage, and guard your pieces against it; particularly guard your queen and king against the zwischenzug. This tactic is all about forcing your opponent into making tough decisions. Your opponent has to ask themselves, for example: "Do I take his knight with my bishop, or guard or move my queen?" These kinds of situations will generally provide you with a tempo advantage as your opponent retreats.