Bracing involves contracting both your anterior abdominal muscles and your lower back muscles at the same and bracing like you were about to get punched in the stomach. For even more stability you push your abdomen out as this will engage your obliques and produces even more stable and stronger contraction.
Good midline stability allows success when performing functional movements, and increased capacity in those movements. Engaging your midline when performing any movement will also reduce the chance of any training relating injury and hence keep you on the track to hitting your PRs.
Be sure you master this first while performing any movement before you start adding any weights.
Next time you hear “KEEP YOUR CORE TIGHT” or “SQUEEZE YOUR ABS” remember this post and what are you meant to do!!!