Presets are saved messages you can load in one click. The Manage presets window is where you build and rearrange them.

Choose Message > Manage presets. A tree opens with your categories and the presets inside each one. From here you can add, edit, move, or delete both presets and categories.
The buttons to the right of the tree manage its contents:

If you have a message ready in the composer, choose Message > Add message to presets. A Create preset dialog opens, pre-filled with the current title, body, and message type, with the name left empty. Type a preset name and click OK, and the preset is saved into the first category. You can move it to another category later from Manage presets.