What it means
A UTM is a few plain-text parameters added to the end of a URL. There are five: utm_source names where the link lives, utm_medium names the kind of channel, utm_campaign names the push, and utm_content and utm_term separate two versions of the same thing. Your website analytics reads them and reports the visit under those names instead of lumping it in with everything else.
Why it matters to a small business
Without them, most of your traffic arrives labelled direct, which means nobody knows. Tag the link in your email footer, the one in your Instagram bio and the QR code on the counter card differently, and you finally get to see which of them actually sends people, rather than guessing.
The bit people get wrong
They are case sensitive and completely unforgiving. Facebook, facebook and FB will be reported as three different sources for the rest of time, and nothing will warn you. Pick a lower-case convention on day one and write it down. The other thing worth knowing is that they sit in the address bar in plain sight, so a customer can read them and so can anyone they forward the link to: never put a name, an email address or anything else personal in one.