An alternator does charge the vehicles battery while the engine is idling and as long as the alternator is turning the battery will be charging some.
The battery gets more charge though when the vehicles engine is spinning faster and you're going down the road or highway.
But you will get some charge from the alternator even at idling speed.
When you jump start a vehicle you should let the vehicle idle for around 5 to 10 minutes before taking off and drive the vehicle around for awhile and at highways speeds to let the alternator charge the battery back up before shutting the vehicle down again.