Pretty normal stuff. The engine, when started, runs at a higher rpm so it can warm up. During this short period, the fluids are being circulated and warmed up. You can drive the car right away, but go easy so you avoid damaging any parts.
It should settle into the normal idle within 30-45 seconds.
This is especially appreciated in cold areas (or winter, for those of us with seasons) because the car warms up pretty quickly and blows heat shortly after start up.
Don't worry - everything is fine.