Largest Contentful Paint records the moment the biggest visible element in the viewport finishes rendering. In practice that element is a hero image, a video poster frame, or a large block of heading text. LCP answers the question a visitor actually cares about, which is when the page looks loaded.
Google's threshold is 2.5 seconds at the 75th percentile of real visits. Between 2.5 and 4 seconds needs improvement, and beyond 4 seconds is poor. The percentile matters: a fast median with a slow tail still fails.
Four costs make up the measurement. Time to first byte, then the time spent loading render-blocking CSS and JavaScript, then the time to load the LCP resource itself, then the time to paint it. Profiling which of the four dominates is the first useful step, because the fixes are different.
The common wins are unremarkable and effective. Serve the hero image in a modern format at the size it displays, preload it, remove the lazy-loading attribute from above-the-fold images, and move render-blocking scripts out of the critical path.