What does the different SEO web vital metrics mean?

Page speed score 

The page speed score is calculated from the other web vital metrics and creates an understandable KPI about your page's performance. 

Grading: 

Good: > 90 

Needs work: 50 - 89 

Poor: < 49 

FCP - First Contentful Paint 

FCP is the time it takes the browser to render the first piece of content after a visitor visited the page. In simpler terms, it's the time before the visitor sees anything at all on your webpage. 

Grading: 

Good: < 1.8 seconds

Needs work: 1.8 seconds - 3 seconds 

Poor: > 3 seconds  

FID - First Input Delay

FID is the time it takes between the first time a visitor interacts with your page (not including scrolling) and the time for the resulting action to occur within the browser. Basically, how long it takes before the visitor's action is performed. 

Grading:  

Good: < 100 ms

Needs work:  100 ms - 300 ms 

Poor: > 300 ms 

LCP - Largest Contentful Paint 

LCP is the time it takes for the largest text or image is rendered on the visitor's screen above the fold. Plainly, the time before the largest content on the page is visible to the visitor. 

Grading: 

Good: < 2.5 seconds 

Needs work: 2.5 seconds - 4 seconds 

Poor: > 4 seconds 

CLS - Cumulative Layout Shift 

CLS tracks the visual stability of the page and the movement of already visible elements while loading.  If elements shift places as they load in, that's going to negatively affect your CLS score. 

Grading: 

Good: <  0.1  

Needs work: 0.1 - 0.25 

Poor:  > 0.25