Hi is a common greeting, more casual than "hello." When you walk down the street in a small town, it might seem like everyone you pass says "hi."
The word hi is a good, nearly all-purpose greeting — it's appropriate in all but the most formal circumstances, so you might want to skip saying, "Hi" to the Queen when you meet her. When you greet someone, you have a lot of words to choose from, including "hey," "hello," "hiya," and "howdy," but hi is probably the most common. The first mention of hi attributes it to a Kansas native American in the 1800s.