A stable left side during impact and on your follow through would see a position where your left shoulder, left hip and left foot were all directly above each other, with 70% of your weight on your left foot, at impact.
If you stood next to a wall and doorway, you could achieve this position by swinging away from the wall on your backswing and then practicing moving to the wall with your left hip and shoulder by your impact position. Notice how your left foot cannot turn towards the wall as it is set up parallel to the wall and your left hip and shoulder can move up to the wall but not through it. This is referred to as a stable left side.
However, this is not helpful if you want to hit actual shots, so try this drill. Take your set up position and place a golf ball just on the outside of your left foot. Swing away from the ball on your backswing and as you swing down, keep your left foot in place so that you do not move the golf ball, but step your right foot over your left as you hit through the shot. This will keep your left foot planted and form a stable left side as you hit through the golf ball.
Do this working on keeping your balance as you land your right foot and you will be hitting more consistent, accurate and longer shots in no time.