https://dragonage.fandom.com/wiki/Abilities_(Inquisition)
From that site, which is a pretty good info resource on the game if you wish to browse it....
Focus abilities are represented by diamond icons as they are also active abilities. They are color coded to be yellow and deep brown. These abilities are only unlocked after the first act of the game and are able to be used when enough focus has built up. Focus is a resource gained each time someone in the party deals damage. Each party member has a personal focus meter, but all party members gain focus when a single member deals damage. The focus meter has three tiers, the first of which is unlocked automatically and the two higher tiers can be unlocked using Inquisition Perks toward Advanced and Master Focus in the Forces category."
Basically this is what we have been saying, Focus is a thing that is built and then used up when using a skill that requires it. Once you have used it, you have to build it back to be able to use the skill again. This is why the skills that require Focus are so strong, the balance is you can't use them often otherwise they would be massively overpowered.
Sorry, I don't know where in the codex it explains it, but the demo of the first focus ability should have been a walk through on focus...when you were escaping Haven just before the song.
