A rajah is a king or ruler of an Indian state, especially in the older kingdoms of South Asia. You also see the spelling raja. In English it is mainly a history-and-story word now: you meet it in books about India, in old travel writing, and in tales, not as the everyday title of a leader in today’s news. Though actually, the rank was not always “king” in the European sense — some rajahs ruled small states, and under the British Raj many were kept as princes under British power. English borrowed the word and then used it a little loosely for Indian royalty in general. It is not slang, but it is not a daily-life word.