Gerrymandering is a method by which the boundaries of council wards are moved to create the majority desired by the current authority. It was used widely in Northern Ireland by the loyalist authority during the 1920s-30s in order to keep the nationalists (who were generally Catholics) subdued, hence stopping them from gaining a political foothold which could have led to the destabalisation of the Northern Irish state.
I am not aware of it being expressly illegal (I study History GCSE, not government and politics), but I imagine such an action would be taken to make elections more fair, gaining equal representation of each sect within the population to allow equal civil rights for all. It would also make elections more competitive, preventing one political party from abusing their power to remain dominant in a country, despite public opinion as a whole.
It makes elections unfair because if there is a slight majority of supporters of party A in one part of a city (for example), the boundary of the electoral wards could be moved to subdivide it into minorities compared with supporters of party B in other areas of the city, whereas a whole this area would have easily voted-in a party-A representative.
I hope this helps, although it is quite a tricky concept to explain in detail.
