Yes, but your park allowance depends on the pass. If you purchased a Base Pass for one park, you may leave and re-enter that particular park as many times during the same day as you wish. If you chose to go with the Park-Hopper option, you can enter multiple parks on the same day, so you’re less limited.
Some people choose to seek reprieve from the heat by taking a lunch or hotel pool break during the hottest hours of the day. Those folks often come back in to the parks to enjoy the afternoon and evening festivities, such as the parades and fireworks. If you do this, don’t forget your glow sticks! They’re pricey in the park.