Gerrymandering is tinkering with legislative district lines by political parties.
Gerrymandering boils down to rigging a district in one party's favor. It screws over the other party and more importantly, the citizens who represent that other party, whose vote basically becomes worthless in each election.
As an example, one political party can gerrymander or redraw the district lines so that it includes more citizens who are representative of that political party's member base. The benefit? That political party now has more voters than the other party thereby making it much easier to win congressional seats.