The Focus Orb is an element within the main game screen that shows the current target. A "target" can be a NPC, item, interactive element of the environment, party member, or Hireling, or even yourself - the term "target" doesn't mean you will (or even can!) attack it, just that you're interested in examining or interacting with it. You target something by left-clicking on it with your mouse pointer, or you can cycle through possible available targets by using Tab ↹.
The buttons on the Focus Orb will change depending what possible actions you can take with that target. Above the Focus Orb is a d20 that lists your most recent die roll.
The location Focus Orb (and all elements of the main screen) can be re-arranged and customized, and then locked again, by using the key.
Collection of generic focus orb images. These are useful to fill most item pages, as Turbine almost always re-uses these for new named items.