A hopper ticket allows you to go back and forth between both Disneyland park and Disney's California adventure parks in the same day as much as you want
As already said, you can go between both parks in the same day during an unlimited times during the day. To add some additional exposition to this: It is difficult to really see all of both parks in one day, if not impossible. Hopper tickets are great when visiting multiple days or if you are familiar with both parks and have a specific plan for one day.
If you have a kids go Disneyland if you have a grown kids go to adventure
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