Bhagavad Gita, Chapter 6, Verse 19

As a lamp in a spot sheltered from the wind does not flicker, even such has been the simile used for a Yogi of subdued mind, practising concentration in the Self.

A steady mind doesn't mean an empty one, it means one that's stopped being pushed around by every passing gust.

The lamp image is precise, the flame is still lit, still doing its job, it's just not flickering wildly. A disciplined mind works the same way, fully active, just no longer at the mercy of every distraction.