“Sneaker zoom” means changing your framing by physically moving closer to or farther from the subject instead of changing focal length. It’s commonly used when talking about prime lenses like an 85mm f/1.8, since the lens itself does not zoom.

A more common phrase is “zoom with your feet.”

Important difference: moving the camera is not the same as zooming. When you move, perspective changes because your shooting position changes. With a real zoom lens, perspective stays the same if you keep the camera in the same spot and only change focal length. If you want a tighter composition with a fixed lens without changing perspective, the closer equivalent is cropping rather than walking forward.