Core Web Vitals are UI Metrics designed by Google that measure the overall quality of user experience on your site. They assess things such as the appearance of content, interactivity of the page and visual stability from the moment of page load. Core Web Vitals are gathered from real world usage data of a website (hence some smaller websites that haven't been well sampled may not return an appropriate result). Google has made Core Web Vitals a ranking factor for pages with increasing importance.
To improve your Core Web Vitals scores, you may need to read Google's documentation on the topic and follow the recommendations provided in the Google's PageSpeed Insights assessment.