Basically, they’re talking about the use of magic with no limits or restrictions. It’s a plot killer, because if your characters can use magic whenever they want, however they want, with no negative consequencers or drawbacks - then, what exactly are you writing about? Your characters have conquered their fate, so to speak. Nothing is standing between them and what they want, because they can use magic to clear away the obstacles no problem. And that is boring. Conflict drives story. Magic without consequences subtracts the conflict. Forcing your characters through the motions of struggle at that point doesn’t make sense, because the whole time, your reader will be thinking, “Well, they can just use magic, right?” Even mortal peril will be unconvincing.
TL;DR Magic without consequences = deus ex machina = story death.
If you want your characters to use magic, but still want your story to be at least kind of believable, then you’ve got to impose restrictions. There are a number of ways you can do this. I’ll use Harry Potter since it’s familiar: In Harry Potter, underage wizards are not permitted to use magic outside of school, and if they do anyway, they’ll face expulsion and may never be able to use magic again. Wizards that are of age can’t use magic around Muggles (except in certain emergencies) lest the Muggles uncover their existence. There are also in-universe enforced laws on how you can use magic - killing people with magic is illegal, for example. Ditto mind control and torture. And the magic itself is limited. You can’t conjure up food out of nothing, as Hermione famously chastised Ron in Deathly Hallows.
Your characters may need extensive training before they can use magic. It may drain them of energy, so they can only use it in a real pinch and not again until they’re restored. Certain types of magic may be illegal, or too dangerous to perform without extreme precautions and advanced knowledge. The magic itself may be unpredicatable, it could have a mind of its own and only work when and where it wants to. It could be limited to very small changes. Maybe the only people who can use magic are women.
Magic can and should help your characters achieve their goals, but if you’re using it to give your characters a free pass to do whatever they want, then it’s a problem.
I hope that helps!