Tip Of The Iceberg Meaning

(idiomatic) Only the beginning; just a small indication of a larger possibility; a problem is much bigger than it seems

Example:   This is only the tip of the iceberg. Our problems can become much worse.
2014, A teacher, "Choosing a primary school: a teacher's guide for parents", The Guardian, 23 September 2014:
  Sats don't reflect the bigger picture; rather the tip of an iceberg in the only subjects that politicians think matter.