There are a lot of individuals who exercise but they do not focus on working their opposing muscle groups. For example, they do a lot of triceps exercises, but only do a few biceps. Another example, is they focus too much on their quadriceps and not enough on their hamstring muscles. Even though the quadriceps will be more powerful and stronger than the hamstring group, you don’t want the balance ratio to be so far off that you strain or tear a hamstring muscle (sprinting, running, etc.)
The goal is to achieve balance and to keep your body in the best alignment as you can possibly get it in. When muscles groups are completely imbalanced, this creates spinal alignment issues , increased strains & sprains in muscles groups and a higher rate of injury.
Remember, focus on having a strong body and look at working out as a way to strengthen the body as an entire unit, versus just one muscle group. You can still split the body up when you are training, but do it in a smart & systematic way. Doing 20 sets of chest and 5 sets of back is not the way to go.
Good luck and have a blast in the gym!