I've written a few chapters of a novel and believe I've used a third person shifting POV, but some readers tell me I'm head hoping. What is the difference?

Basically, how often you’re shifting POV.

The more often you shift perspective, the more likely you are to confuse the reader. If you shift for each chapter, no one is going to notice. If you shift inside a paragraph, the reader is likely to do a double-take or get confused as to who is doing the thinking. See, the “rule” is there for a reason: “never confuse the reader”, unless of course you’re deliberately being misleading so as to not give away whodunnit on page 3 of a murder mystery (or if you’re writing a script for the TV show Dark, in which case “confuse the viewer” is basically what it’s about).

And just as with everything else: with enough skill, you can get away with anything. Don’t assume you have the skill. Terry Pratchett, famously, did a lot of head-hopping, and he always got away with it. If you want to use this technique, read his works and try to figure out why it worked for him. Find others, and see why it worked (or didn’t). (And if you want to confuse the reader, watch Dark and figure out why they got away with it. You’d better watch it through twice first just to understand what is happening, and start on the “got away with it” bit later. It’s that sort of show.)