Your “core”, as it is called in general terms, is the key to optimal performance, stability, and posture. It is the foundation for all movements of the body, and also functions as a biological lifting belt and brace.
Being able to activate your core will improve your posture by keeping your spine in alignment. It will also improve your ability to balance and perform complex movement patterns. Trying to perform technical and/or compound exercises without first activating your core is the easiest way to get injured at the gym, especially as you increase the weight being lifted.
To learn how to properly activate your core we urge you to speak with your personal trainer so that they can walk you through the steps in person. They may even give you simple core activation exercises that you can do at home or at work.