When things are printed, there's typically a pass through a roller made for each colour. In this, there'd be red, blue, green, and black. When something's off register, that means that one (or more) of the colours is no longer printing where it should. On the examples above, the most obvious indication is that the red "dot" in the centre of the white flowers is right in the bottom, but in the top one, it's off centre, moved a little up and to the right (as these photos are oriented.) This means that as the fabric had all the "red" printed on it, something was out of alignment, and it didn't print correctly.
Usually, if the manufacturer catches it, they'll either scrap the run or sell it as "seconds" - to a discount fabric store or thrift store, usually. Sometimes they don't catch it, and you end up with part of the fabric just looking a little funny.
This happens in all manner of print shops - I notice it most often in grocery flyers, the kind printed on newsprint, where they don't have the time or money to scrap the run, but just go with it.
As an aside, one of the reasons there are "dots" of each colour used in the print along the selvedge edge of the fabric is to ensure proper alignment - so if you're questioning as to whether a fabric is "right" or not, check the selvedge edge and if the ink is perfectly within the circle (or square) then it's fine, but if one colour is way out of the lineup, pass on it.