What it means
Email tools measure opens by putting an invisible image in the message and counting when it loads. That worked well until mail apps started loading those images automatically, on their own servers, whether or not a human ever looked at the message.
Why it matters to a small business
Apple's Mail Privacy Protection, which arrived with iOS 15 in 2021 and is on by default, pre-loads images for Apple Mail users. Those opens are recorded whether the message was read, glanced at, or deleted unseen. Since a large share of consumer email in the UK is read in Apple Mail, an open rate is now a blend of real interest and automatic loading, and you cannot separate them.
The bit people get wrong
Never A/B test a subject line on opens alone any more. The winning line may simply have gone to more Apple users. Judge subject lines on clicks and on replies, and treat opens as a rough trend against your own previous sends rather than a measurement.