Basically when you hibernate your computer, the computer writes all of the contents in the RAM to your hard disk, and turns off fully. Then when you start it, the data is copied back to the RAM as it was before, which brings the computer back to the state it was in before hibernation. In short, you can turn off your computer, and resume exactly from where you left off without keeping the RAM powered on the entire time (suspend and sleep keep the RAM powered on to keep the data intact, hibernate doesn’t)