A clinical trial is an experiment that involves patients in a new way of managing a condition. This might include investigating a new treatment, a new way of giving an existing treatment, or a new approach to diagnosing an illness or assessing an outcome after treatment.
Clinical trials are run because there is belief that the new way may be better than the standard way, and trials are vital to establish whether this is so. There is no guarantee of this, however - clinical trials are experimental by nature and there is a chance that the new way will be no better, or not as good even, as the standard way. Unexpected side-effects are also a possibility, or you may be put into the 'control group', which receives a placebo (dummy drug) or the standard existing treatment, rather than the new treatment.