Yes, a 1-Park Per Day ticket to one of the Walt Disney World theme parks allows you entrance into one park and you may come and go as many times as you please on that same day.
Likewise, a 1-Park Per Day ticket at the Disneyland Resort entitles you to enter and exit that park without restrictions. There is no additional charge for leaving the parks and coming back.
When using a Walt Disney World Park Hopper Ticket or Disneyland Park Hopper Ticket you can come and go into any of that resort's theme parks throughout the day. Many guests return to their resort for a mid-day break to take a nap or swim at the resort pool.
Additionally, if you've paid for parking at a theme park in the morning and then return with your car later in the afternoon, you won't be charged a second time on the same day.