This information comes up on the first run to give an explanation of Oplop. If you need to access it again in the future, you can do so from the "Help" item in the application menu.
Using a single master password and various nicknames, one can create an infinite number of unique account passwords. These unique account passwords are commonly called password hashes, domain-specific passwords, or per-site passwords. The How It Works page on the Oplop Wiki explains what Oplop is for in more detail. A general overview of the available documentation on the Wiki can be found in the Table Of Contents.
The Oplop project aims to provide multiple implementations of its core algorithm so that one's account passwords are available at any time. This is just one of many implementations. Oplop is even available as a website. Since Oplop uses an algorithm to generate the passwords, you can use your nicknames and master password with any of the implementations and get the same generated password.