ElvUI will let you rearrange all of those elements so instead of being on the edges of the screen they're closer to the action, allowing you to see everything more easily.
While the basic World of Warcraft UI is usable, taking the time to install a full overhaul like ElvUI is a smart idea that will help you during difficult dungeons and raids. For one, the basic UI spreads necessary elements like your action bar, health, and monster health all over the screen. When you're desperately trying to attack, dodge fatal abilities, and see the status of the monster you're fighting all at the same time, it can get overwhelming. ElvUI will let you rearrange all of those elements so instead of being on the edges of the screen they're closer to the action, allowing you to see everything more easily.