Macros let you spit out a wad of text by hitting a key combination. This pane lets you create sets of macros.
To create a new macro:
Click on the "+" button. You'll see a dialog asking you to type the key you want to use for the macro.
After typing the key, the dialog will vanish and you'll see the new entry in the list of macros.
Double-click on the line of the new entry, in the area underneath the Action column. The area will turn into a text entry field; type the text you want Cantrip to send when you type the macro key. Note that the text will be sent as a complete command to the MU*, instead of being put in the Input Area.
To delete a macro, select it in the list, and click the "-" button. To turn it off temporarily, uncheck the checkbox for the macro.