Stationary means not moving. A stationary bike is the kind you find in the gym––you pedal, but the bike stands still. When troops are stationed somewhere, it means they're not on the march––they're staying in one place.
Do not confuse stationary with stationery, which is paper and other supplies used for writing. Just remember that the one with the "a" comes first in the dictionary, and you must sit still for a minute thinking before you can begin to write.
