In the example below we can see that this sites core web vital scores are in the green for mobile FID and CLS and Desktop FID.
I've highlighted the core web vital over all score which shows the average score for the last 28 days. This time period is a rolling one so as you make changes this score will reflect this over time.
The squiggly line is a representation of the average score for each day over the 28 day period. We can see in the image below that this site about a week ago has had a significant change in their core web vitals. As the 28 day period rolls forward their overall score will improve to reflect this.