You have now targeted a potential adversary. The color of its health bar, located at the top of the screen, tells you how difficult it is to fight it.
Grey opponents are easy, green could be a challenge, yellow are difficult to beat and opponents with red health bars are opponents you should NOT attack if you are alone, as they will probably kill you.
If you want to attack this opponent, press Q on the keyboard, or use the Start Attack shortcut on your shortcut bar, to initiate attack. Use any special attacks you might have, such as Brawl, Fling Shot or Full Auto when you need to. They have a refresh time, so you may want to use them early on.