Time to First Byte (TTFB) measures the latency between the browser firing a navigation request and the first byte of the response body arriving. It captures everything that happens before rendering can begin: DNS lookup, TCP handshake, TLS negotiation, redirects, server processing, and the network round-trip.
TTFB is not a Core Web Vital but it's the floor under every loading metric -- a slow TTFB makes great FCP and LCP scores impossible. Google's recommendation: under 800ms on the 75th percentile of real users.
Improve TTFB by caching rendered HTML at the edge (Cloudflare, Fastly, Vercel ISR), eliminating slow database queries on the request path, putting the origin geographically closer to users (multi-region or read replicas), and minimising redirect chains (each redirect adds a round-trip).