The anomalous expansion of water refers to the fact that water, unlike most substances, expands when it freezes. The density of water increases as temperature decreases, but reaches a maximum at 4 degrees C then begins to expand. The significance of this is that ice is less dense than water so it floats in water.
This is of major biological importance. Lakes, ponds and streams freeze from the top down instead of from the bottom up because ice stays at the top. Ice doesn't conduct heat very well so the layer of ice keeps much of the water below from freezing. This allows fish and other aquatic life to survive cold seasons. If this was not the case food chains would be disrupted and aquatic organisms would likely become extinct.