The armoury saves a record of the skills, gear, attributes, champion points etc. you currently have equipped, so you can reload them later on, without having to pay for respecs and without having to manually select everything.

The menu itself is very simple, if I remember correctly there's 1 button to save what you're currently using, 1 to switch to the selected template (once you've got 1 saved) and a 3rd to rename a template.

If you've already got a build you like the best way to use it is to save that build, then switch to another one and save that, then switch to the 3rd build and save that. When you're setting up the templates you will need to pay for respecs, but once you've got a template saved switching to it will be free.

If you don't have 3 builds one popular use for a slot is to respec everything back to zero and save that, so you can use it as a free respec to change your character to whatever you want (especially useful if you like experimenting with builds).

A few limitations to be aware of:
1) The armoury doesn't store gear, to be able to swap to it you have to have it in your inventory.
2) It saves everything every time, there's no option to have a template that will only change your champion points for example, but it is possible to save 2 which are identical except for the champion points so you can switch between them.