Question: I am a little confused about the difference between tight third person and omniscient. I am writing a young adult historical fiction and I would appreciate any advice you can offer. Thank you so much and I am grateful for your help in the past as well.
The difference lies in whether your narrator describes things that are outside your main character's perception.
An omniscient narrator knows everything about the story. He or she can report what's happening anywhere and what any character is thinking or feeling at any moment.
In tight or limited 3rd person, your narrator can only describe what your main or "point of view" character is aware of - her thoughts, her feelings, and what she sees, feels, hears, smells, tastes, etc.
You cannot, for instance, describe what is going on in any other character's head. The reader (and the main character) can only figure out what other characters are thinking/feeling by what they say and what the main character notices about their actions, facial expressions, etc.
Similarly, you cannot describe anything outside the main character's range of perception. No description of what's happening around the corner, out of earshot, or in a different building. You can't show what other characters are doing when the main character is not present.
It's a little more challenging to write in limited 3rd person, because you sometimes have to find creative ways for the main character (and thus the reader) to find things out - conversations, messages, inferences, etc. However, the advantage is that the readers get to experience the story in a very intimate way. They can enjoy the illusion of being the main character.
It's your choice what type of narration to use, however omniscient has largely fallen out of favour in recent decades. You will see it sometimes, but not often.
However, if you choose to use 3rd person limited narration, you must stick to it. You must be careful not to break the illusion by occasionally switching to omniscient. That would be like an actor breaking character. It spoils the experience for the reader.