It's an intensifier that can be used with adjectives, adverbs, and verbs to emphasize their meaning.
Examples:
* Mzuri sana: Very beautiful
* Haraka sana: Very fast
* Nimefurahi sana: I am very happy
* Nina upendo sana kwako: I love you very much
It's a very common word in Swahili and is used in many different contexts.