What is a declaration number for?

The DCN is the tracking number for your return. It tells when, where, and who filed your return for tracking.

So it doesn't mean your refund has BEEN approved and funds are on the way?

No, the number is assigned when filed, before being approved. Remember, if your refund includes Earned Income Credit, Additional Child Tax Credit, or Stimulus - those refunds will take longer to fully process. Your refund could even come in two parts