The Counter-Strike fanbase likes to say that their game is really complex and has some hard mechanics to understand. This actually is the case whilst comparing it with some other big FPS titles. In CS:GO for instance, you have to stand still while shooting with most weapons. There are multiple ways to go about this, with the easiest one of course being not holding a movement key anymore. Your character will slow down and eventually come to a halt. If a dynamic crosshair is being used, it will stop moving and come to rest the moment you stand still.
The harder and more efficient way is ‘counter-strafing’. To do this you have to counter the movement you are already making by pressing the opposite key. For instance, if you are moving to the right, you have to press the left key to counter that movement. At a certain moment you will come to a halt. You can just keep hold of both keys to stop and remain stopped, but you can also release the key of your prior movement and tap the counter-key to stop just that a little bit quicker. This is heavily timing-based though, so it has a higher skill ceiling and a greater chance of failure.