The endpoint in a clinical trial is an event such as occurrence of a disease, or symptom, or a particular laboratory result. Once someone reaches the endpoint, they are generally excluded from further research in the trial.
In contrast, a hard endpoint is an endpoint that is well defined and can be measured objectively. For example in cancer research, the endpoint in a trial might be related to response to treatment (such as shrinkage of a tumour).
The endpoints used in a trial must be defined and documented as part of the trial design.