A glacier in balance will not change shape. This means that the amount of snow falling at the upper parts of the glacier equals the amount of ice melting at the margin. However, glaciers are never completely in balance. During summer there will usually be much more melt than snow fall and during winter much more snow than melt.
During summer, glaciers will retreat due to more melt than snow fall. During winter the glaciers will advance due to more snow fall than melt. Even though the glacier is advancing in winter and retreating in summer, you still say that it is in balance if it ends up at the same place every year.
Watch this video showing one year in the life of a calving glacier.