Moodle's filters read text and media that the users put on the site. The filters can then do three things with that material: link, interpret, and/or restrict.

These filters can now be enabled site-wide by the administrator, disabled site-wide, or be turned off by default with the option of the teacher enabling them in individual courses.

A filter can automatically link words and phrases to items in your site. For example, suppose you create a Glossary that contains the phrase self-determination. If you activate the Glossary Auto-linking filter, whenever that phrase appears on your site, it will be highlighted and will link to its glossary entry. When a reader clicks on the phrase, the reader is taken to the glossary entry.

Secondly, a filter can interpret what you have uploaded. For example, you could upload a document that is written in the markup language called; TeX (think HTML on steroids). The TeX Notation filter would interpret this document and enable Moodle to display it correctly. There's also an Algebra Notation filter that interprets a special markup language for writing math formulas.

Finally, a filter can restrict the kind of content that a user can place on the site. For example, the Word Censorship filter can filter out a list of bad words that you don't want to appear on your site. Every time that text is uploaded or entered, it is checked against the list of forbidden words.

You'll find the Filters settings under Site administration | Plugins | Filters. Read the following descriptions for detailed information about what each filter can do for your site.