Intensity tells you how hard you worked in a single workout, so your Intensity over time graph gives you an overview of your past Intensity Scores
Your main Intensity Score is giving you a summary of how you performed in your recent Live Activity. If you want to see how your Intensity has varied over your last couple of weeks then you need to look at your Intensity over time graph.
On a mobile you need to navigate to your Intensity Section from your main Dashboard. At the bottom of that section you will see the below summary which you can open for more details by clicking on the blue arrow in the top right of that tile.
By opening that you will see the below chart. This chart is automatically open on tablet and PC/Desktop.
You can see how your overall workout Intensities have varied over the weekly period you have selected. In general, you want to see high scores, meaning you have been working hard, but Intensity is something that should reflect your training plan and your intended intensity levels. In a build-up or tapering period, it would be expected to see an upward or downward trend overtime here.