Generally speaking, this is an Arabic word meaning approximately "struggle" or "trial," but which is often euphemistically used to refer to a war. Specifically, the word is used often to describe the civil conflicts of the first few centuries of the caliphate, especially those enormous chaotic conflicts to which the rulers sometimes had to resort to settle multiple simultaneous succession disputes and rebellions. For more information concerning individual conflicts, see the following nodes that I spent a large amount of time writing:

First fitnah

Second fitnah

Third fitnah

Fourth fitnah