Why it breaks aim
Aim is muscle memory for distances: move the hand this far, turn this far. Acceleration makes the distance depend on speed as well, so the same 5 cm swipe travels further when you do it fast than when you do it slowly. A quick flick and a slow correction of the same length stop agreeing with each other, and consistency is exactly what you are trying to build.
Where it hides
In Windows, the setting is called Enhance Pointer Precision. Despite the reassuring name, it is acceleration: it scales cursor distance by how fast you move. Mouse software can add acceleration of its own, and some games carry acceleration settings too, so check all three.
The mouse settings guide walks through turning it off.
How raw input sidesteps it
Games that read raw input take movement straight from the mouse and skip the Windows setting entirely. AimBetween does the same in the browser: when a round starts it captures the mouse with the Pointer Lock API and asks for unadjusted movement, which takes the operating system's acceleration out of the loop.
A quick test that it is off
Put the cursor on a corner of an icon on your desktop. Move the mouse slowly a few centimetres to the right, then move it back to the same spot on the mousepad quickly. With acceleration off, the cursor returns to exactly where it started. With it on, the fast return travels further than the slow outward move, and the cursor ends up past the starting point.
In a game that reads raw input the desktop setting no longer matters, but the test still tells you whether your mouse software is adding acceleration of its own.