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A filter is a Drupal feature, usually implemented by some contributed modules, which substitutes parts of a node text with some other text.

Eg: if you use the Path filter module you can insert links like:

<a href="internal:node/25">hello!</a>

and the string internal:node/25 will be substituted by something like http://www.example.com/node/25 where www.example.com is your website url.