Triple oppression is a theory developed by black socialists in the USA. The theory states that there exists a connection between the various types of oppression. It hypothesizes that to overcome the oppression you must strike at all types of oppression concurrently. The original three types of oppression mentioned were capitalism, racism and sexism.
An example of triple oppresion in practice is a factory that contains both men and women of various ethnic backgrounds. The workers cannot unite to fight capitalism and obtain better working conditions, because they are hamstrung by their own racist and sexist beliefs.
Some people say that triple oppression has now been expanded to further types of oppression such as homophobia and speciesism. This idea is also held by many anarchists,as explained here: http://www.infoshop.org/faq/secB1.html#secb14