I saw the term used when describing the Canon 85mm f/1.8. What do they mean?
It means the lens focal length doesn't change, but you can change the framing by moving your feet — which in this idiom are wearing sneakers (which, for non-native English speakers, are casual athletic shoes). I've more commonly heard "zoom with your feet", which is the same thing except not dependent on shoe style.
It important to be aware that changing your position is not the same as zooming, because moving alters perspective, while zooming does not. To get the effect of zoom with a fixed-focal-length lens, don't move — crop.