What is the idiomatic meaning of scratch surface?

"Scratch the surface" is an idiom that means to only explore a small or superficial part of a topic, subject, or problem. It implies that there is much more to discover or understand than what has been revealed.

Here are some examples:

* "We've only scratched the surface of this issue. There's a lot more to learn."

* "This research has just scratched the surface of the potential applications of this technology."

* "My trip to Paris was amazing, but I only scratched the surface of all the things I wanted to see."

Essentially, it means you've barely begun to understand or experience something, and there is a lot more depth and complexity to it than you initially realized.