Systematic reviews gather ALL the relevant studies on a specific clinical question and synthesize the best studies in order to give the best evidence answering the question.
Quality of Evidence Pyramid
Systematic reviews provide one of the highest levels of evidence and make recommendations to guide clinicians and administrators to make decisions about policy and best practices.
They're called "systematic" because they use pre-defined rigorous, methodical, and clearly documented methods in each step of the research process. This reduces bias and ensures that the research is transparent so that readers can assess their quality.