Rotten boroughs were districts in England in the early 1800s in which relatively few residents had the power to elect a representative with the same vote as much bigger districts. This gave the residents of the rotten boroughs greater relative voting power.
For example, Newtown (located on the Isle of Wight in Britain) had been a thriving market town but by 1832 it consisted merely of a village of 14 houses. Worse, there was no secret ballot then, so it was common for one man, the patron, to control all the votes.
In the Reform Bill of 1832, Parliament redistricted the cities to eliminate rotten boroughs to reflect the growing and shifting populations.