I'm assuming you've decided against an Omniscient POV for your story. Your story is going to be related by the characters as they see it unfolding before them, much as it happens in real life. If Joe is having an argument with Helen, he can see what Helen is doing, he can hear what Helen is saying, but he doesn't know what Helen is thinking or how Helen is thinking. He can make guesses as to what Helen is thinking by what she says and how she says it. He can make guesses at to what Helen is feeling by how she looks as she says and does things. But he doesn't know with 100% certainty what she's feeling or thinking.
So, when you're writing from Joe's POV you can have him guess at Helen's thoughts, but not know them.
Helen's cheeks flared red as she spat out the words, 'I hate you.'
That's neutral description, what a character can see and what a character can hear. From that, it's reasonable for Joe (and the reader) to guess that Helen is cross, but neither Joe nor the reader know for sure.
Joe thought Helen looked furious as her cheeks flared red and she spat out the words, 'I hate you.'
You could even write Joe knew Helen was furious because her cheeks flared red etc as the 'because' shows that Joe is reasoning it out using the same clues that the reader has.
What you can't do is this: Joe didn't know what to say or do. Helen was furious. Her cheeks flared red as she spat out the words, 'I hate you.'
But you could do this: Joe didn't know what to say or do. Helen looked furious. Her cheeks flared red as she spat out the words, 'I hate you.'
Headhopping is when you jump from one character's head to another and back again.
Joe didn't know what to say or do. He flapped his hands ineffectually.
Helen was furious. Her cheeks flared red as she spat out the words, 'I hate you.'
Joe felt his jaw tighten in response as he stopped himself shouting back. One of us has got to have some self control, he thought.
'Don't you have anything to say?' Helen shouted. Why wasn't Joe arguing back? Didn't he care any more?
Joe took a deep breath. Stay calm, he told himself. 'Look, Helen,' he began to say but she cut him off.
'I've done enough looking. I'm leaving you.' She picked up her bags, registering with a flicker of pleasure his horrified expression.
The effect of headhopping to the reader is similar to being a spectator at a tennis match, you're going back and forth between the two characters. It's exhausting and you risk confusing the reader fairly quickly.
At which point I'm pretty certain you're digging out your favourite author and showing how they headhop all the time. It happens. It used to happen a lot, but times change and you see headhopping less often now. I think we're much more aware so have become sensitive to it. Speaking as someone who gives feedback regularly, I think headhopping is the easiest thing to spot and comment on. Any agent or editor who reads your work will notice it too.
Headhopping is something that's as easy to fix as it is to spot, so writing that features headhopping suggests that badly edited work by a writer who isn't in control of their craft. The reality of today is that work that gets published is work that doesn't need a lot of editing.
Some people seem to never head hop. Other people do it all the time. I've no idea why there should be this difference but if your nature inclines to headhopping then you need to take steps to edit it out - or accept that it make make publication harder (tho not impossible).