Explanation of Z and X
You can use the keys on your keyboard Z and X in order to click the circles to the beat as an alternative to clicking on your mouse or tablet, whilst still aiming by moving a mouse or a stylus. It gives more precision, and you can aim better with it. You will be able to FC (full combo) more maps if you make the switch to clicking on your keyboard and aiming with your mouse.
You can also change the keys Z and X to anything else on Options -> type in “change” -> click “change keyboard bindings”. Change left click or right click to any two keys you want.