Cold-blooded aquatic animals like fish typically enter a kind of hibernation in the winter in order to preserve their energy. Fish will congregate around the bottom of the pond where the water is slightly warmer. Here, they'll stay still and rest so that they use fewer calories. Consuming food during this time can be dangerous, as it can restart their metabolism during a time when body functions like digestion can't be maintained. Plant life will often wither away and then grow back from dormant seeds once temperatures begin to warm back up.
While fish will enter a period of dormancy and won't require food, oxygen is still a necessity to sustain life during the winter. If your pond ices over in the winter, you'll need to ensure there's always a hole in the ice that allows the air to make contact with the water and permit an exchange of gases: harmful gases can escape, and oxygen can dissolve into the water column. Gentle bubblers can be used to increase contact between the air and the water and ensure that oxygen is available throughout the pond.
