Stationary means not moving. A stationary bike is the kind you find in the gym –– you pedal and pedal, but you just don’t go anywhere.
Something stationary is staying right where it is, like that bike, or anything else that doesn’t budge. If your car crashes into a stationary van, it’s probably your fault because that van was parked. If your aunt is sick and in the hospital and has been that way for a while, the doctor might say her condition is stationary. Animals that don’t migrate are stationary. Troops that aren’t marching are stationary. Something stationary is still.