Turtles are ectothermic animals. They need to be warm to move around and eat. If they get too cold, everything in the body will slow down. Then they can't eat and digest food. The heart slows down and the lungs do too.
Some turtles live where it is warm all the time.
They stay active all winter long. But turtles who live where it gets cold and snowy have a way to keep themselves alive. They can hibernate . Different kinds of turtles hibernate in different ways. Most water turtles go deep into the pond and snuggle down into some mud and leaves at the bottom.