Rotten boroughs were one of the curiosities of the British electoral system. Rotten boroughs were a product of a system that did not want change, where fathers passed on constituencies (and the power as a MP that went with this) to their sons as if they were property (which many saw them as), where some rotten boroughs were so bizarre that they beggared belief and where the very few who voted could not vote for whom they wanted to due to the lack of a secret ballot or challenging candidate.
The 1832 Reform Act swept away these constituencies. Three of the more famous were:
Dunwich was a coastal village in Suffolk. By 1832 much of it had collapsed into the sea but its 32 voters still returned two MP's to the House of Commons.
Old Sarum had, in medieval times, been a thriving town. However by 1832 it had seven voters living in three houses but returned two MP's to Parliament.
Gatton in Surrey had just 7 voters and returned 2 MP's
Clearly such a system was ripe for reform and the 1832 Act disenfranchised 56 constituencies labelled as rotten boroughs. After the act, the voters who lived in these former constituencies had to vote as part of a county electorate and the former constituencies lost all independent legislative representation as they were absorbed into new constituencies that better reflected the population increase in the United Kingdom.
Rotten boroughs that were disenfranchised also included: Aldeburg in Suffolk, Castle Rising in Norfolk, Gatton in Surrey, East Grinstead in Sussex (now West Sussex), Yarmouth in the Isle of Wight and Orford in Suffolk.
Many of the 56 rotten boroughs had existed for many years and had become part of the political 'scenery'. To many in the Tory Party especially, they were seen as a political right to those families that 'owned' them.
Therefore when Grey targeted rotten boroughs, he was seeking to remove something that many MP's took for granted - especially those who had seats in the Commons via a rotten borough.