What does The tip of the iceburg mean?

The beginning of something much larger, which may be mostly unseen or unknown. It means there is a lot more to something than you might originally think, just as most of an iceberg lies beneath the surface of the ocean and is not able to be seen.
ice is lighter than water so it floats, but only small part is above surface.

The saying means that some much bigger problem is hidden behind what is already visible/surfaced.
Most of an iceberg is under water. What you see is the top, which is the tip of the iceberg. So there is much more of it than you can't see. So the idiom is used when there is much more to something than you already know or can see. Someone is telling you about a problem. You their problem is bad. In trying to tell you that there is much more to the problem than they have told you so far, they may say that "That is only the tip of iceberg." So the problem is far worse than you already think it is, based on what you know so far.