What it means
A small piece of code on your website, or your own customer list uploaded to a platform, lets you show ads specifically to people who have already been in contact. They know who you are, so the advertising has far less work to do.
Why it matters to a small business
It is usually the cheapest advertising a small business can buy, because the audience is small, warm and already interested. For anything people think about before buying, a wedding cake, an extension quote, a course of treatment, the gap between looking and deciding is where the booking is won or lost.
The bit people get wrong
The audience is tiny, and that is the trap. Spend a strangers-sized budget on a few hundred past visitors and you will show the same person the same ad several times a day for a fortnight, which turns a warm lead cold. Cap how often it shows and let the audience refill.