Headhopping is when we switch between the perspectives of characters mid-scene. It's confusing for readers and can throw them out of the story.
Headhopping generally happens when a writer doesn't stick to POV guidelines. Here are some tricks I use in some of the more troublesome styles.
Third Person Multiple
In this POV we spend an entire scene or chapter within one character’s perspective. In the next scene, generally, we switch to the POV of another character. In each scene we see the world through the eyes of that particular character, and no one else. This includes the character’s internal feelings and reactions. A good example of this is Game of Thrones.
A quick trick I use to keep from headhopping in third person multiple (and third person limited) is to think of it as writing in first person:
"I threw a twenty on the table."
Only swap out the personal noun for something more distant.
"Mike threw a twenty on the table."
"He threw a twenty on the table."
Since you're locked to one perspective in first person it is impossible to headhop.
In this POV the story is told by a narrator who knows the inner thoughts of the characters. The narrator should be written as a character his or herself, and have a strong voice. Hitchhiker’s Guide to the Galaxy is an example of the Subjective Omniscient POV done well.
Think of it as a grandfather telling a story to his grandkids. His “voice” should come through in the text. You'd be in the grandfather's POV the whole time, so any headhopping between characters wouldn't feel unnatural.
In this POV the narrator is all-knowing but doesn’t have a voice and is unable to relay the inner thoughts of characters. A distant, unbiased observer.
I tend to think of this POV as “movie-mode” because when writing I imagine a camera filming the action. We can see what the characters are doing and saying, but we don’t know what they’re thinking—it has to be guessed through physical cues and dialogue.