While exercising, you break down the body, fortunately, we have a built-in mechanism that rebuilds us in a slightly stronger version than before while we rest. Therefore, we must get the rest we need after physical exertion. This is called SUPERCOMPENSATION and is the basis of all progressive training.
In the drawing below, your shape is basically the horizontal line in the diagram. The first training period consumes the body's resources, and this is illustrated by the fact that the green line goes down. Then you have the much-needed recovery phase/rest period with good healthy food so that the body can be rebuilt and reach a higher level than when you started. This is about finding the right time when you have recovered enough, and then start training again. This is how the super-compensation runs and you constantly get up to a higher and higher level.
Below, the appropriate training (supercompensation) is illustrated again at the bottom left and to the right, the inappropriate training (overtraining) is illustrated. When overtraining, you can see that the recovery time after the first workout (the red line) is not long enough. Ie. that the person in question starts training before the body has rested completely. This continues from there and all the time the level comes further and further down to the starting point. The person breaks down the body more and more, becomes more and more tired and before long there are injuries, etc.
There is one more training mode, and it is illustrated at the bottom of the two you just read about. It can be called "maintenance". Ie. that after your workout you keep a too long rest period before you start again. This means that your super-compensation has time to drop to the starting point again before you start training. That way, you do not really get anywhere - you maintain your level.