The tip of the iceberg describes a situation where only a very small part of something much larger is visible. A dictionary is just the tip of the iceberg when it comes to the millions of words in the English language.
This phrase comes from the way icebergs float in the ocean: Only about 10 percent of an iceberg's mass is visible above the surface; the remaining 90% is hidden underwater. Saying that something is just the tip of the iceberg is a bit like a warning: If you don't look below the surface, you might get caught off guard by the sheer scale of what you're dealing with. Think of the Titanic, which was taken down by a seemingly insignificant iceberg!