Keywords are a list of words/symbols/patterns that when found in the body of a post or article will be replace with a link and text of your choice. To add keywords, you must login to your blog and select the Options tab >> Keywords.
Some of the features/configurations of keywords:
Case sensitive
Replace first instance only
Open link in new window
Ability to set a tooltip/href title
There is one caveat to mention. All current versions of .Text are hardcoded to only use Keywords the first time a post is saved (whether it is mark as published or not). I was afraid of adding a feature that could not be turned off. With the new configuration storage options, .Text will be getting much more flexible you should be able to explicitly configure this option.