Hi Colleen,
I have great news for you!
You'll be an official Disney Guest on your vacation! In "Disney speak," there are two kinds of people at the Walt Disney World Resort. A Guest is simply anyone visiting the Walt Disney World Resort, while employees (called Cast Members) are the ones who magically make Guests' vacations special and memorable!
As far as who is allowed to enter Main Street U.S.A. before the opening of Magic Kingdom Park, it depends on the day. All Guests will be welcome on Main Street U.S.A. prior to the official opening and may even enjoy some shopping and dining, but it isn't always an hour ahead of time. That time may vary by day. I would definitely encourage you to arrive at Magic Kingdom Park at least 30 minutes ahead of time if you're arriving by Disney transportation and closer to an hour ahead of time if you're driving and parking at the Transportation & Ticket Center. By arriving early, you'll have plenty of time to go through security and ticketing and be ready to catch the enchanting
Let the Magic Begin welcome show before the park opens.
Now the other thing that you could have read about is
Extra Magic Hours. Every day, Guests who are staying at a Disney Resort hotel will be invited to enter one of the four theme parks one hour early or stay two hours late, enjoying the same magic and fun with smaller crowds. If you're staying at a Disney Resort hotel, you'll simply need to scan your MagicBand to participate. Extra Magic Hours are definitely a wonderful perk!
Thanks so much for visiting us at the Disney Parks Moms Panel, Colleen, and please let us know if you have any other questions as you're planning your trip. I hope that you have a truly magical vacation!
Missy