"You're off and running."= you have made a start and you are progressing very well. The sentence comes from the official start of a horse race when the announcer calls, "They're off and running!."

This is usually said when someone has begun a task and made good initial progress from the start. "Have you started on your book report." "Yes. I've finished the first page." "Good! I see you're off and running!"

It could mean that somebody is off to a good start. It could also be used for the future of course. 'Pull out the spark plug, clean the contacts, put fresh gasoline in the tank and you're off and running' (or or 'up and running' 'ready to go') It is mostly used for the future when giving somebody instructions on how to do something.

Reduce it into two parts: "off" = you have left the starting line (figurative expression) "running" = in motion

I associate the phrase "They're off and running" with horse racing to describe when the horses have just left the starting gate and are running down the track. Similarly, you can think of a person as just starting an enterprise or a project. The speaker who says, "You're off and running," thinks you're off to a good start.