Keyboard completion inserts templates into your emails by typing a shortcut, then pressing a keyboard combination (by default, the Tab key).
For example, if you have a "Kind regards" template with the "kr" shortcut, you would type "kr" in the email body then press Tab. The template text will then be inserted in your email.
You set each template's shortcut when you create the template.
Disabling Keyboard Completion
If you want to disable keyboard completion, you can do so from the Settings page.