As with third person limited point of view, in Omniscient Point Of View there's a disembodied narrator who's telling the story, and the characters' actions are described by saying he, she and they.
The difference is that the reader is in all the characters' heads all the time, and knows what they're all thinking at any given time. (The word omniscient literally means all-knowing, and that's exactly what it is.)
An example might make this clearer:
John laughed hollowly. “You’re joking,” he said, wondering how on earth he would ever get over this.
Veronica shook her head slowly. Her heart was breaking at having to tell him this news.
John stood up and banged his fist against the wall, hard, once, but that did nothing to disperse the fury coursing through him. He still couldn’t believe it. “I’ll have to leave now,” he said, thinking that he couldn’t bear to stay there another moment.
Veronica nodded slowly. He was upset now, but she knew he'd get over it.
You see the way we knew what both characters were thinking, even within the same scene?
I imagine this POV having the "camera" floating around the ceiling somewhere - able to see all, even into everybody's thoughts.
The one advantage of it is that there's a lot more knowledge for the reader. They, too, are all-knowing.
There are two big disadvantages, however:
The first is that the reader has a broad view, not an intimate one. He or she can't identify particularly with any specific character. There's a big danger of losing the reader's interest as they just won't care what happens because they're not feeling empathy with any specific character.
The second disadvantage is that it can be very jarring and disconcerting, particularly if you start off in one person's thoughts, and suddenly announce what another character is thinking. This knocks the reader out of their reading trance - which is absolutely not a good thing.
Now, many writers use omniscient point of view, including very respected writers such as Gabriel García Márquez, so who am I to say?
The fact, however, is that I don't like it at all. I find it hard to read novels using this POV, and indeed won't read them unless there's a compelling reason - I'd never read one for pleasure.
I know, too, from writers' forums, that I'm not alone in this opinion.
And so I think I'm safe in saying that no agent or publisher ever turned down a novel for not being written in omniscient point of view. No reader rejected a book on that basis.
Third Person Limited Point of View is not going to offend anybody, and allows you as much flexibility as Omniscient, and I strongly recommend you use that POV in preference to omniscient.
Writer's Digest offers over 1300 writing tutorial videos. You can buy individual lessons, or access them all on subscription. Here are some of the videos they have on point of view: