In the name of Allah, Most Compassionate, Most Merciful.
Hasan literally means good.
Technically:

A hasan hadith is one which has a connected chain of upright narrators, who have good retentions, and which is free of any irregularities or defects.

Difference between saheeh and hasan.

The only difference between a saheeh and hasan hadith is the level of the narrator’s memory.

In saheeh it is excellent, while in hasan it is only good.