Gerrymandering is unfair because it is designed to manipulate elections rather than allowing a fair and unbiased vote. Politicians redesign districts to ensure a heavy majority for themselves and their political allies, ensuring their party stays in power.
Gerrymandering generally only happens every 10 years. This is when the census is taken, and valid reasons for redistricting may result. Each district is supposed to have a fairly equal amount of voters, so as populations change, districts must be redrawn. However, when politicians redraw the lines to fragment the opposing party's voters, it prevents those voters from having an equal say in the political process and rigs elections.