It's the plural form of "casita", which means "small house".
The word is often used to describe:
* Small houses in a larger compound or resort: For example, you might find "casitas" on a ranch or in a luxury resort.
* Tiny, stand-alone dwellings: These might be used as guest houses or as homes for the very poor.
* A small, separate structure within a larger home: This might be used as a studio or a home office.
The word "casitas" is often used in a more casual and affectionate way than the word "casas" (houses). It can also be used in a more whimsical way, suggesting a charming and cozy space.