This question can best be answered by looking at the counterpart, a private good. Private goods are typically traded in markets. Buyers and sellers meet through the price mechanism. Thus private goods tend to be excludable. For example, others cannot enjoy a piece of cake, once consumed.
Public goods have just the opposite qualities.
An example is a street sign. A traffic light or clean air is a further example. This poses immediately the question of who, then, provides public goods. Once they exist, they are there for all to enjoy. So it is often the most rational strategy for private actors to let others go first and seek to enjoy the good without contributing to its production.