Gerrymandering is a practice of drawing voting districts in a way that creates an unfair advantage for whoever happens to be drawing the line.
Using computers politicians are able to redraw lines with scientific precision to include or exclude whichever voters you want to include or exclude in a district.
Lawmakers often reshape voting districts to shift the balance of political power. That's unfair to voters, even those of us with questionable judgment.
Every 10 years we have a census after which district lines are redrawn, the problem is the lines are redrawn by politicians who have a vested interest in protecting their own power.