Filters are functions that can be used to narrow the search results of a search query.
On online databases/catalogs, filters are usually offered at the Advanced search function. Filters that appear at the search functions of practically every online database are title, author, publisher, publication date and type of publication (article, book, dissertation, etcetera). The range of other filters tends to vary per database.
Search field specification
In advanced search, often it is possible to search by several fields at the same time. This means that you could for example request a certain title by a certain author. Below you will find an example of the use of multiple search fields (author, title, abstract and full text) in the search interface of JSTOR - advanced search: