To put it simply, Joy-Con drift is what happens when you move your character on screen one way, but the character moves a different way. A more bookish answer is that Joy-Con drift happens when an analog stick on the controller deviates from the central neutral position, leading to unwanted player input.

Why is joy-con drift a thing?

Joy-Con drift is when Joy-Con register input where there is none. This means your character, or a reticle, could move in a direction without your input. This problem was first reported early on in the Nintendo Switch's lifecycle.

What Does drifting mean on a controller?

Joy-Con drift is an issue that Switch owners have been encountering over the last few months that causes the analog sticks on the controllers to randomly move around and input commands to the console, even when they're not being physically moved.