Let's talk about head hopping!
Head hopping is where, in a 3rd person POV, the narrative point of view (or the head we are in) jumps between characters within the same scene or paragraph. This is primarily a thing in closer 3rd person POVs where we are more in a character's head--we are getting their thoughts, feelings, and experiences directly.
Here's what head hopping can look like (two excerpts from the book Sacred Fate by Eressë Belley):
Imcael sighed. Tyrde had his work cut out for him. He wondered if his youngest was up to the challenge.
It was some time before Rohyr rejoined his consort. Not an offensively long time but it was clear to astute observers that here was no love match. Even Rohyr’s attire bore testament to his lack of enthusiasm for the occasion. He was clad in the customary nuptial colors of cream and gold but there was little of the resplendence expected of the ruler of a great realm. Only the heavy collar of white-silver links and square-cut adamants marked him as royalty. And in lieu of his crown or even a ceremonial coronet, he wore a plain gold circlet.
But the most telling sign of all was the way he periodically twisted the wedding band of entwined gold and white-silver on the middle finger of his right hand. Not in the manner of one unused to wearing a ring on that finger but rather as if it were too tight and he wished he could remove it.
After a few minutes conversation with Tyrde, Rohyr scanned the hall until he spotted Lassen. His leman was seated at one of the far tables. Engaged in conversation with Eiren Sarvan and Reijir Arthanna’s brother Keiran, he showed no sign of unease though he must have felt it at a feast celebrating his lover’s marriage to another.
In the span of four consecutive paragraphs we go from Imcael's head to a vaguely omniscient POV to Rohyr's POV, with no clear transition between them. Just a few paragraphs before this, we had been back in what might have been that vaguely omniscent POV, which had this rather jarring transition:
If it galled him that said correctness was born out of protocol rather than true amicability toward Rohyr’s betrothed, he held his tongue. After all, they did not know Tyrde. For now, all his son could command was deference to his office to be. Loyalty would come later.
The shift from the vaguely omniscient POV (which includes a speculation of Imcael's feelings) directly into Imcael's head happens mid-paragraph, making the reader jump from having no direct insight into Imcael's thoughts to being in his thoughts with no transition.
My goal is not to pick on this book in particular (I chose it in part because I had already talked about its head-hopping-ness in my April/May reading challenge update) but to show some of the issues that can arise with head hopping:
It can be confusing or hard to follow and make for a jarring reading experience. The reader can end up never quite sure whose head they are in.
It makes the story more disjointed. A POV switch is often a natural transition point in a story and forces the reader to recalibrate for the new POV, and rapid switching breaks the story up at unnatural break points.
It makes it less clear who knows what and/or what information is true. As with the sentence that I quoted above, we shift from speculation as to Imcael's thinking directly into Imcael's thinking in one paragraph. As a reader, I would generally take speculation as something that can't be assumed to be wholly true--but immediately being in Imcael's head makes it seem as though it must be true. But it was speculation from a vaguely omniscient POV--were we in someone's head? Whose head? It may not always be clear the literal sentence when the POV switch happens, leaving the reader unclear whether information in any given narration happened in one POV versus the other.
There is no one set standard for when to switch POV in third person, but I tend to find the most successful versions switch POV either after a clear scene break (which is quite common in dual-POV romance novels) or at a new chapter. Either way, you will generally be best served by having a clear transition point and something that quickly makes it obvious whose POV we are now in.