TLS replaced SSL. The names are often used interchangeably ("SSL certificate" = TLS certificate in 2026). The current production version is TLS 1.3.
A TLS certificate proves the server's identity. It's signed by a Certificate Authority your browser trusts. Let's Encrypt is the most common CA in 2026 — free, automated, valid for 90 days. ngrok, lrok, and Cloudflare Tunnel all serve real Let's Encrypt certs at their edge.
Many browser features require HTTPS regardless of whether you'd technically need TLS encryption: service workers, geolocation, getUserMedia (camera/mic), Web Push. localhost is exempt for most of these — but your phone, your colleague's laptop, and OAuth providers all see your network address, not localhost.