Tell me what's a grade 2 sprain?

It depends. There are several different grading systems. Some are supposed to apply to all joints while others intended to apply to certain joints only. However, most have 3 different grades where grade 1 is the most mild and grade 3 is the worst. So, a grade 2 is right in the middle. What that means regarding treatment and prognosis cannot be answered without knowing more info.
Ligament Injury. Grade ii (second degree) injury is more severe than a grade i injury and indicates that the ligament has been more significantly damaged, but there is no significant instability.