When you first install a Munin node it will try to install as many plugins as it can so that it can report on different things. For example, if you have a Varnish server running then Munin will detect this and enable the plugins so that it can report on the activity of Varnish. Once you have started getting data through to your Munin server then you can turn on plugins on the nodes to get more data.
The data of any plugin is presented in a standard format and so is understood by the Munin server. Perhaps the most important plugin for my work is the Apache status plugin that shows what is going on inside Apache. This plugin isn't always installed with the Munin node and so you might have to do this yourself. This is a good way of getting familiar with Munin plugins.