brightn'ove
cringe
There is a whole book that can be written about it. The short version is that the main effect is to change the magnification in a non-linear manner as you focus. Nearly all lenses have this effect (the lack of it is one of the things that make really expensive lenses cost several £k), and the effect is particularly noticeable in macro photography because framing is fundamental. What you'll see is that as you focus, the image will "jump" to a different size suddenly - irritating.
Many casual photographers claim not to be able to see the effect in normal usage, but if you do macro, it's pretty hard not to see it - something that was very nearly in focus and filling the frame, will suddenly be pin point sharp, but no longer precisely framed. In the worst case, it can lop off 10% of the previously framed subject.
To summarise further, it's when a lens unintentionally 'zooms' in and out as you focus.