Fundamentalism is a strict interpretation of the scripture, like Protestant Christians who believe that all the miracles in the Bible really happened. Islamic fundamentalism is the same, except people believe everything in the Koran actually happened.
The word fundamentalism was originally associated with a Protestant movement of the early 1920s, which protested the treatment of the supernatural miracles and events of the Bible as anything but literally true. Fundamentalism was created in direct opposition to modernism, a movement that sought to reinterpret the events of the Bible based on modern times. Nowadays, fundamentalism usually refers to religion, but it can also be a strict and literal belief in anything.