Page speed is how quickly your website shows up and becomes usable when someone opens it, from the first blank moment to the point they can read and tap without waiting. It is the umbrella term that a handful of more precise measures, like page load time and the Core Web Vitals, all roll up to.
For a small business, speed quietly decides two things at once. Visitors leave pages that make them wait, often before a single word loads, so a slow site loses customers you already paid to attract. And Google factors speed into where you rank, so a slow site is found less often in the first place. Speed is felt worst on a phone, on a normal connection, which is exactly where most of your visitors are (see mobile page speed).
You can measure your own in Lighthouse, the free tool built into Chrome. It will tell you, in seconds, what your visitors feel.
I build sites to load fast on a real phone, not just a fast laptop, because a page someone waits on is one they may never actually see. Speed is a floor to clear, not a finish to chase, and nearly every other term in this glossary sits on top of it.