A fort is usually a place that either is or was used as an army post! Forts are often built with strong, protective, surrounding walls and other structures that help with defense, like ditches or trenches and posts for a lookout.
Have you ever been to a place that had “Fort” in its name? It’s very common. Lots of cities and neighborhoods whose names begin with “Fort” were once used by troops as their station! (Sometimes, the place might even still be used that way!) You can often visit the fort in these special places to see all of its neat features and learn some of its history!