Upper crossed syndrome is a muscle imbalance where the chest/pecs become short and tight and the upper back muscles (trapezius, levator scapula) become lengthened and tight. Most of the time we feel the symptoms in the upper back - knots, pain, tightness.
Our first inclination is the stretch the upper back out, but those muscles are already stretched and overactive. The solution is to strengthen the back muscles and stretch the chest muscles out.
One of the biggest things we need to do is work on our daily positioning or posture. We can exercises all day to fix the problem but if we continue to fall into bad posture the issue will continue reoccurring.