"Head hopping" is moving from a focused third-person narration with one character's point of view to a focused third-person narration with a different character's point of view. It's supposed to be one of those Bad Things that automatically make a text Bad Writing. It's usually cited in the same breath as "always use 'said'", right after "show, don't tell". And, like a lot of other writing rules, it gets broken in popular, canonical books all the time.
Virginia Woolf head-hopped in both To the Lighthouse and Mrs. Dalloway. Philip K. Dick used it to brilliant effect in several books, including (my favourite example of all time) Radio Free Albemuth, where not only does he hop between different heads, but he breaks point of view mid-sentence and then picks up the same sentence from the point of view of a different character. Stephen King even head-hops for a few sentences at a time in Hearts in Atlantis before switching back to the main narrator's point of view.
So what's with the Universal Writing Rule of "No Head Hopping"? I think it's similar to the what John Wiswell found when he did an analysis of prologues in popular books. Many agents and publishing industry types insist that including a prologue means an instant rejection because "everyone hates reading them," yet on the list of thirty-six books in John's blog post there are several nominees for major awards.
If head-hopping is done well, it's considered structural experimentation by a brilliant writer. If it's not done well, it's considered a stupid newbie mistake, which somehow elects it to be graven in stone somewhere in a list of Things Writers Shouldn't Do.
I would respectfully suggest there should be another stone engraved with Things Critics Shouldn't Do, and that one of the items listed near the very top should be "Don't Confuse the Device with the Execution." Just because it's difficult doesn't mean it should be banned.