WHAT ARE CORE WEB VITALS?
Google’s aim is to make sure that all the websites included in their index are offering the best experience for their searchers. By making Core Web Vitals a ranking factor, Google is leveraging search visibility to encourage website owners and developers to focus on UX. 

In short Core web vitals are just not important for SEO but they are User Experience(UX) standards set by google to make your visitors experience wonderful in your website.

What does it mean to not pass the Core Web Vitals assessment?
If your site does not have a passing score on the different Core Web Vitals metrics, signifies that your website has poor user experience. Having strong UX is critical for ecommerce sites in particular; those that take a long time to load, have slow-acting CTA buttons, or page content that jumps around are problematic now and repercussions will increase with the Core Web Vitals rollout. Having a strong user experience, and, therefore, passing the Core Web Vitals assessment can help increase conversion rates.

Google has created tool to check your core web vitals, which helps to generate the report of your core web vitals.