What does an animal hibernating mean?

Hibernation is when an animal goes to rest for the winter season to stay warm. Not all animals hibernate.

Hibernating results in a deep sleep over a period of time that limits bodily functions. Many forest animals such as bears, deer, owls, and other warm blooded animals stock up on fat by eating tremendous amounts of food. Once they have enough body fat to keep warm and survive during the winter season, they hibernate.

Only one animal is known to actually go into a deep sleep and not hibernate, the grizzly bear - its bodily functions do not slow as happens in hibernation, but it has special bidy functions instead.
Hibernate means that a animal stocks up on food and then sleeps throughout winter.
Hibernation occurs in mammals like bears and squirrels during the winter, where they store food in the fall, and then sleep throughout the entire winter season, and wake up in the spring. Otherwise, the mammals wouldn't be able to find their food in the winter, and hibernation makes them sleep and not get hungry, and slows down their heart rate and the need for food untile spring.
Some species of animal hibernate during the winter, meaning they enter sort of a low-cost sleep state. They metabolize nutrients stored during the fall more slowly, keeping a lower body temperature, and breathing more slowly. It's a way to conserve energy during cold weather, instead of eating more in order to combat the cold.
Hibernating means when an animal sleeps in winter and wakes up when the weather gets warmer.

when it gets to cold some animals such as bears,snakes,salomanders and more go to sleep in winter and awake in spring. The animals basically store food in body to last during winter. It is a whole process of temperature. Many animals do not hibernate because they can survive the long winters.