It's stands for
Couch to 5 km and is designed to get a non runner from the couch to running 5pm in 9 weeks. A new runners biggest mistake is going too hard too fast so it is a gentle introduction. Tonight I ran for 60 secs walked for 90 seconds with 8 runs altogether and by week 9 I'll be running for thirty minutes straight. You do it 3 times a week. It's pretty easy to follow.