In practice
It is a server and network metric: database queries, uncached templates, slow origins and distance all land here.
A CDN fixes distance but not a slow application — measure both edge and origin before choosing a remedy.
Why it matters
Every other loading metric starts counting from it, so a slow TTFB caps how fast a page can possibly be.
It is also the metric most visible to crawlers, and sustained slowness reduces how much they will crawl.
Related terms