Learn your basic anatomy. This will help you understand where in the body the muscles are, what the muscles are supposed to perform, so when you train, or when your trainer tells you you are training quadriceps, you know exactly what he or she means.
It's important because it helps you learn your body better, so that you develop your own personal set of tools to take with you for the rest of your life.
This is important because if you have a bad week, you can wake up one day, when you are ready to start, and remind yourself how to train a certain muscle group, so that when you go to the gym and train, you are able to get right back to it.
Front Arm Muscles- Biceps Back Arm Muscles- Triceps Chest Muscles- Pectorals Back Muscles- Middle Trapezius, lower trapezius, rhomboids, latissimus dorsi, Shoulder Muscles- Upper trapeizus, anterior, medial and posterior deltoid Front Thigh Muscles- Quadriceps Back Thigh Muscles- Hamstrings Back Hip Muscles- Gluteals
There are obviously many more muscles than this, but these are the basics you should know about.