Critical mass is the minimum amount of something required to start or maintain any project or venture. In order for a new ice cream store to succeed, the need for it must reach critical mass.
Any movement or venture has to reach critical mass to carry on and succeed. The critical mass of a popular band would be the point at which their popularity starts to explode. In nuclear physics, critical mass is the smallest mass of material that will start a chain reaction in a nuclear bomb or reactor. Both kinds of critical mass begin chain reactions.