what is meaning of clinically significant endpoint?

Clinically Significant Endpoints are defined as the desired result which determine that treatment is worked. In other word, it is the clinical outcome of interest.

This is the main result that is measured at the end of a study to see if a given treatment worked (e.g., the number of deaths, or the difference in survival, between the treatment group and the control group). The primary endpoint is decided before the study begins.