It will very likely come as a surprise to many people that bees have to be fed. "Feed bees! But, don't they eat honey and pollen?" Yes they do but, if it was a long winter they may need a little help if their supplies of honey are a little low. Apart from that some people take too much honey and put the bees under a little too much pressure. The hive may go into decline and need feeding to bring it up again. If the hive is short of honey for the winter the entire colony may die or at least lose many bees.
The colony may decide to leave in search of better conditions.This is virtually unheard of in winter (bees avoid flying in winter) but could happen in the summer when the weather is good and the bees may have some chance of finding a better spot. In order to deter this you might feed them. It is also a way of giving them their medicine.
Hives should be inspected from time to time during the winter. Do not open the hive because the bees are "wrapped up" against the cold and disturbing them will not do them good. They will have to bring the hive up to its operating temperature again. This will use up much of their food supply. In winter the only bees to fly are ones that are defecating or cleaning the hive. Clean little guys, aren't they?
The bees maintain a hive temperature of 35° when they have brood. It will be less than this during the winter, but it still needs to maintain a comfortable range for the bees to survive. Even if disturbed they may survive but in a weakened condition. If you see bees out foraging and "looking hungry" put some food near the hive. If they need it they will take it. If not, it will still be there when you next visit.
So what do you feed bees? Sugar water. A little water and lots of sugar. Keep putting sugar into the water un till it stops dissolving. Also make sure that your bees have enough water all year round. If your hive is near a residential area this is very important. Because the bees may be inclined to go into kitchens looking for water. This will not endear you or your bees to anyone. So, a little pond near the hive or a shallow dish that you keep filled. It needs to be shallow or have bits floating in it for the bees to land on. Bees can not swim and will die if the water is too deep.