In software terms, a filter is an algorithm that sorts through data in order to find relevant results. For example, if you type in “apples” into a search engine, the search engine will go through all of its indexed web pages and filter them to return only pages that are relevant to the search term “apples.”
In WordPress, however, filters work a little differently. In WordPress, filters are one of two kinds of hooks. Instead of sorting through data, a WordPress filter finds data and then modifies it according to instructions contained in the hook.
Note: If you’re only planning on operating a WordPress site, then you won’t need to every worry about what filters are or how they are used. For developers, however, filters and actions (the two kinds of hooks) are used for building themes and plugins.
To see a filter in action that is included with all WordPress installations, create a new post. The URL or post slug that is generated for that post is the result of a filter. WordPress will take the plaintext title of your post and “filter” it to create a URL with no spaces that may contain the title of your post as well as the date on which it was written.
The WordPress Foundation manages an extensive (but not exhaustive) list of available filter hooks on its website.