The limit break is fully explained when you hover over it.

When you first interact with a limit break, an Active Help window appears which explains it as well, including how to navigate to Actions & Traits to put it on your hotbar, with a picture.

What it doesn't explain is when to actually use it. SE can't really tell a tank not to use their limit break ever. You also have a choice between using the melee limit break on a boss or an AoE limit break on trash and in my opinion, it doesn't matter. We are so beyond the item level of content, that things die before even reaching LB2 in the first place.