The theory behind critical mass is that car drivers are like the dinosaurs. Cars are going to die out. We need to stop driving and using fossil fuels to survive on this planet.

A 'critical mass' is a group of people whose numbers have reached that critical number where new ideas/consciousness start to spread like waves to the entire populace.

So the cycling critical mass hold demonstrations taking over traffic to 1 show car drivers that their days are numbered, 2 to show others that cycling is growing in popularity 3 that people riding care about the environment/planet by cycling instead of driving, and they are hoping others will start caring too.