What is the Small String Optimization (SSO)?

Let's discover that in the following blog post:

The C++ Small String Optimization

by Giovanni Dicanio

From the article:

How do “Connie” and “meow” differ from “The Commodore 64 is a great computer”?

In several implementations, [...], the STL string classes are empowered by an interesting optimization: The Small String Optimization (SSO).