Sahih ---> means authentic

Hasan ----> means good/ acceptable

These are gradings scholars give to narrations the prophet reportedly has said. Scholars use certain criteria in order to increase the confidence that a hadith was said by the prophet. Just because a hadith is graded sahih, it doesn't guarantee that the prophet said it, and just because it is graded weak, it doesn't mean the prophet didn't say it, rather this is just the apparent judgment WE use to distinguish.

The main difference is that generally if one or more of the narrators is found to have issues with recall/ memory, and they aren't accurate in their reporting, but they are still of sound character it would be hasan, while a sahih wouldn't necessarily have this issue.

Scholars use sahih and hasan as evidence for legal opinions and the like.