What it means
Show version A to half your audience and version B to the other half at the same time, change one thing between them, and let the results pick the winner. Changing one thing is what makes it a test rather than a redesign.
Why it matters to a small business
It replaces the argument in the room. Small businesses lose whole afternoons to whether the button should say Book now or Check availability, and a week of real traffic settles it for free. It also compounds: every settled question stays settled for every campaign afterwards.
The bit people get wrong
Small businesses usually stop them far too early. Forty bookings against thirty-two looks like a clear winner and, at those volumes, is well inside the range you would get from running two identical versions against each other. If your traffic is modest, test big differences rather than button colours, because only a big difference will ever show up.