You may not be familiar with the term RRD graph, but if I show you one, you probably recognize them instantaneously. They are used to plot all kind of data against time in a very easy way which is why they are used a lot in all kind of applications. Even though many consider RRD as a library to create graphs, it is actually more than that: it's a complete system to store aggregated data in a very efficient way.

He talks about how to load in the extension (or the command line, if it won't compile) and create a sample graph or two from some database information. The functions the extension enables make it simple to create some basic graphs quickly and easily.