Commons refers to a shared resource that can be used by all. If everyone uses it appropriately, it can last. But few people tend to over-exploit it - making it run out. This is called the Tragedy of the Commons.
A classic example is a common pasture land. If all the shepherd only use it well, it will replenish itself. But if there are greedy shepherds who want to protect their own lands, they will only graze on the common land - running it out.
The "Creative Commons" phase comes from this concept.