What is gerrymandering and how does the party in control use it to control voting districts?

Gerrymandering is the dividing of a state, country, etc. into electoral districts so as to give one political party a majority in many districts while concentrating the voting strength of the other party into a few districts as possible.

The part in control uses it to control the voting district by "cracking" which means diluting the voting power of the opposing party's supporters across an abundance of districts. Or use it as "packing" which means concentrating the opposing party's voting power in one district to reduce their voting lower in other districts.