It explains one object or process in terms of another. The aptness of the analogy isgenerally a point of subtle judgment. Some analogies are more valid than others. Comparing anapple to the space...
It was just the tip of an iceberg that the RMS Titantic hit and it was enough to cause the damage to sink the ship. That is why when you hear it is the "tip of the iceberg" you know other bad things...