A Systematic Reviews is:
A collation of all relevant research studies in order to answer a specific research question. It uses explicit, systematic methods that are selected with a view to minimizing bias, thus providing more reliable findings from which conclusions can be drawn and decisions made.
They are systematic because they use explicit, systematic methods.
Why are systematic reviews important?
Taken from: The NCCMT. (2016, July 4). NCCMT - URE - Types of Reviews—What kind of review do we need. https://www.youtube.com/watch?v=ep3BRgRJ2N4
The Research Pyramid
The gold standard of evidence is a systematic review: an article which examines, appraises and consolidates findings of all of the primary research for a medical question. Clinical research (filtered) constitutes content that is vetted and verified such as clinically appraised topics and clinically appraised articles.
When evidence from filtered resources is not available, you may need to search the primary literature for unfiltered and/or unverified sources, such as single studies.
The bottom level of information in the pyramid is opinion and background information, the kind of information that may be found in a textbook or encyclopedia.