What is stage separation?
Also called: staging
Stage separation is the moment a rocket drops the part that has run out of fuel. An empty tank is dead weight, so letting it go makes everything above it easier to accelerate.
Falcon 9 separates about 2 minutes 30 seconds after liftoff, four seconds after the first-stage engines cut off.
There are no explosive bolts. Air-powered pushers shove the halves apart, which is gentler and cannot damage the engine that is about to light.
Staging is the single most effective trick in rocketry. A single stage carrying the maximum sensible amount of fuel still cannot reach orbit; two stages comfortably can.
See it happen
Push & GoWhy rockets flySource: Falcon 9 Block 5 mission timeline (Starlink Group 17-38)