Filters are ways to narrow your results. Common filters are publication dates, species, article language, study type, and article type. They may be prominent or available by clicking an "Advanced Search" hyperlink. Some databases also allow users to display results by relevancy or most recently published.
Note: There are cases where you may not what to filter, such as capturing recent publications of a randomized trial your PICO is addressing. If a study has not been tagged as a Randomized Control Study, it will not show in a filtered search of RCTs.