Critical Mass occurs when a certain amount of one or more objects come together at one time (usually crashing into each other), and after they come together chaos reigns supreme. The universe is full of situations where critical mass is required for life to occur.
Firstly, there needs to be enough matter to come together under the forces of gravity for a heavenly body to form. These bodies can be moons, planets, brown dwarfs. Next if there is enough matter in a brown dwarf, the nuclear forces take over, and a star will form.
If the mass of a star reaches a critical mass, it will end up as going super nova, and if there is still enough mass at the end, the star theoretically end up as a black hole.
The human race has found that using the principle of Critical Mass they can develop weapons of mass destruction. Generally nuclear material is mostly harmless unless it is purified and when enough of the purified form is obtained, it can be used in a bomb.
Another form of human created critical mass can be seen in car travel. It just seems that one extra car on the motorway will mean the difference between a nice quick 20 minute trip, or a two hour traffic jam.