What it is
GA4 in plain English
Google Analytics 4 is the current version of Google Analytics, and it replaced Universal Analytics in July 2023. It isn't a new coat of paint on the old product — it's a different measurement model.
Universal Analytics thought in sessions and pageviews. GA4 thinks in events: every interaction, from a page view to a form submission to a purchase, is an event with parameters attached to it. That's more flexible and it's a much better fit for how people actually move between phones, laptops and apps.
The catch is that flexibility only pays off if someone decided, deliberately, what to collect and what to call it. GA4 will happily collect a pile of automatically-captured events and report on them, which is why so many properties look like they're working while telling you almost nothing useful.
Why yours probably isn't right
The July 2023 deadline meant a lot of migrations were done at speed, and many were Google's automated migration, which carried old goals across as conversions without redesigning anything underneath. That leaves you with a property shaped like Universal Analytics, running on an engine that doesn't work that way.
Add to that the things GA4 doesn't do unless you tell it to — filter your own staff's traffic, handle cross-domain journeys, mark the right events as key events, keep raw data longer than 14 months — and you get reports that are confidently, plausibly wrong.