This ususally used relate to politic, sometimes you hear "extreme left" or "extreme right", a discription of a person or a group of peoples' political stand. What exactly does it mean?
The terms 'extreme right' or 'extreme left' are labels that are applied to political groups.
An example of right wing might be fascism and left wing could be communism. However, as with most things they are normally a minority on the fringes. In the UK there are two main political parties - Labour and Conservative. Labour tends to the left and Conservative to the right. Within each party there are groups which subdivide into left and right, with some taking the middle ground. Outside the main parties you have the next biggest group the Liberals (who claim to be a moderate party with some of the ideals of both Labour and Conservative). Then there are smaller, more extreme elements such as the British Nationalist Party (extreme right) and the Socialist Workers Party (extreme left)
There is some information here;
Left-Right politics - Wikipedia, the free encyclopedia