What does MECO mean?

It is the end of the first stage’s job. Everything goes suddenly quiet, and for a few seconds the whole stack is coasting.

Separation follows four seconds later, which is deliberate: the gap lets the engines finish shutting down before the two halves are pushed apart, so the upper stage lights in clean air rather than into an exhaust plume.

You will also hear SECO — Second Engine Cutoff — when the upper stage shuts down having reached orbital speed.