Early extension means that your hips are coming into the ball at impact as opposed to you staying in your posture you had at address. When your left (lead) hip moves forward towards the ball at impact it means that the lead side of your body is no longer rotating . You can't simultaneously move you left hip into the ball and continue to rotate properly.
The lines on the left hip of Tommy Fleetwood (left with an iron) and Nathan Ouimette (right with a driveright) were drawn at address and show how through the golf swing they rotate the lead hip back and away from the ball at impact, not forward and into the ball.
When you stop your body rotation your club path changes and your wrist angles will change.
So check out this quick video where Rob Stanger demonstrates what early extension means and its impact on club path. Notice at the end of the video the lines that you should always be using on your videos to make sure you don't have early extension just like the photo above.