The actual term "tyranny of the majority" was coined by Alexis de Tocqueville, a Frenchman who did a very influential study of American society in the 1830s. However, the idea existed before he came up with the term.
The term refers to the idea that the majority in a democracy can tyrannize a minority. If the majority wants to badly enough, it can take rights away from unpopular minorities whether they be racial, religious, political, or some other type of minority. The tyranny of the majority can be seen in US history in such things as the denial of rights to African Americans and the suppression of communist speech in the 1950s.