Hi!
A park visit is when you visit the Magic Kingdom, Epcot, Disney's Hollywood Studios, or Disney's Animal Kingdom on a specific day. You actually have to enter the turnstiles; just visiting a resort hotel near a theme park does not count towards a visit. With just a base Magic Your Way ticket you can enter one and only one theme park each day. However, you can leave that park in the middle of the day and return. So for example, you could visit the Magic Kingdom in the morning, leave for lunch at the Grand Floridian, and then return to the Magic Kingdom.
If you want to visit more than one theme park on the same day then you need the Park Hopper option. However, you do not need to purchase that option up front. You can wait until you get to Walt Disney World and see if you actually need it. If you actually visit a second theme park on the same day then you can purchase it then. Just remember that the cost of the Park Hopper option is the same whether you buy it on the fist day of your visit or the last!
You can get more information on Magic Your Way tickets
here. Have a magical vacation!