What is a meta-analysis?
For almost every question there are several studies, sometimes with different outcomes. A meta-analysis brings them together.
Systematic review
First, researchers look up all the studies on one question, following fixed rules. That's called a systematic review.
Meta-analysis
In a meta-analysis, the results of those studies are also calculated together. That gives you one overall answer, based on far more participants than any single study ever has.
Not infallible
A meta-analysis is only as good as the studies in it. If those are weak, so is the conclusion.