To slump is to fall or slouch down. It's also a downturn in performance — a struggling ballplayer and a sinking economy are both in a slump.
There are many kinds of slumps, but they all involve things going downhill. If you slide down in your chair, you slump. Other slumps are less physical. If a successful quarterback starts losing games and throwing interceptions, that's a slump. The original meaning of slump, back in the 1670s, was "fall or sink into a muddy place," while the more figurative meanings came much later.