Recently without enabling able to see first meaning full paint value in video compare timing graph can any one explain what it exactly refers?
That’s actually a Chrome metric, not a Lighthouse one (Lighthouse just reports it like WebPageTest does). Here’s a reasonably short explanation of it: GitHub - tdresser/time-to-first-meaningful-paint: A proposal for a Time To First Meaningful Paint specification.
Basically, it is the paint after the biggest layout change in the viewport and an attempt to measure the point in time when the main content displays.