If something is the tip of the iceberg it is only a small, easily visible, part of a problem – there is much more to the problem than is immediately obvious.
Most of an iceberg is under the water, and only a small part of it can be seen above the water.
Examples of use:
1. Her debts are only the tip of the iceberg. We think she might have lost her job as well.
2. I’ve tidied the kitchen but it’s just the tip of the iceberg – the rest of the house is a mess.
Image by kae44