Walkers are huge mechanical vehicles looking like some kind of animal. They walk with four legs and can shoot explosive missiles. There are two different types of walkers. The bigger one on four legs is called AT-AT and the smaller one on two legs is called AT-ST.
Both of these walkers appear in this map. It is a ported map and it seems to be ported to an old world in Minecraft PE as the world is limited to 256×256 in size it seems.