Painting is the last step of the rendering phase for a page load.
This phase takes the box model tree created during the layout rendering phase and positions each pixels accordingly to the screen.
The first painting metrics (FP and FCP) are important to the user experience because it provides feedback that the page is actually loading.
FP marks the point when the browser renders anything that is visually different from what was on the screen prior to navigation.
FCP is the point when the browser renders the first bit of content from the DOM, which may be text, an image, SVG, or even a canvas element. See Page (Load|Speed) Metric - First Contentful Paint (FCP)