For example, let's assume you are searching for articles related to toxicity and remediation. Currently, a keyword search for those two terms (toxicity remediation) in Scopus will yield 5,299 results.
One way to narrow that down is to instead search: toxicity w/6 remediation. This search currently yields only 347 results: a much more focused search set.
Note: As with any filter, you may ultimately exclude useful articles. I like using focused approaches when I am doing my initial searching, and then broaden my search to make sure I have not missed anything.
Specialized Filters (not unique to PubMed, but one of few to offer these options)
Similar to literature reviews, systematic reviews offer an overview of the literature on a particular subject. However, systematic reviews go beyond literature reviews, and aim to provide repeatable analytic methods to collect data (research), and then analyze that data. Systematic reviews will often provide the most comprehensive overview of a particular area of research.
Age / Gender Filters
If you are researching the effects on a particular demographic (age/gender), then PubMed's filters can allow you to quickly narrow your search
Note: PsycInfo is another useful database for limiting your searches to specific demographics.