Filters or Limiters (mean the same thing) are tools that exist in most databases and search engines. The example below is from IUCAT and demonstrates how one can limit/filter a set of results to specific types of content.
Filters do not work directly with keywords, like Boolean Operators do, instead they help limit the type of information being displayed. A general search in IUCAT will look for all types of content including books, articles, different databases, subjects, date ranges, etc.
It might be beneficial to limit your search to specific types of content, which can also vary based on your research assignment. For instance, if you needed to write a book review, you could narrow to look at books only (see example below highlighting formats). To use a filter, simply click on one of the blank boxes, next to its name, then it will automatically limit to those results.