What is the 183 day rule in Spain?

Could you please explain what exactly is the significance of the 183-day rule in Spain? I've heard it mentioned in the context of taxation, but I'm not entirely clear on how it works. Does it have something to do with residency status? If so, how does it determine whether or not someone is considered a resident for tax purposes? Also, are there any exceptions to this rule? I'm quite curious to know more about this aspect of Spanish taxation.