A macro is a programmable function that allows you to do certain tasks at the click of a button/key.
I.e. programming the mouse button to automatically type a phrase.
In FPS games, macro refers to the use of hardware to automatically click at a set CPS (clicks per second) by setting it to a certain time (0.1s) for example. What the macro does is then click every 0.1 seconds the macro button is held, yielding 10 clicks per second.
The effect is more time to concentrate on placement of shots and potentially lower bloom/recoil. Potentially due to the click/shots per second cap on certain weapons.
TL;DR - Too long ; didn't read
Regular macro : If you hold your mouse button, it clicks x times in x seconds.
QuickSwitch macro : If you click once you will automatically type q-q, 2-1, 3-1, etc. so that you quickswitch with one click instead of having to type q-q yourself.