Gerrymandering is the winning side's ability to redraw congressional districts to give themselves an advantage.

In simple terms: Say you have a minority group that votes 'X' surrounded on four sides that vote 'y'. In reality you have five groups, four Y and one X.

The people who control such things redraw the lines, so that the 'X' group is broken up and merged into the predominantly 'Y' groups. The end result would be five 'Y' groups. Basically the 'X' groups votes are watered down.

This is done by all parties, whoever is in power at the time. It is allowed because it is necessary to re-draw the districts over time.

It has no bearing on the Presidential or Senatorial races. Just the House of Representatives and local elections.