check_multi is kind of a wrapper plugin which takes benefit of the Nagios 3.x capability to display multiple lines of plugin output.
It calls multiple child plugins and displays their output in the long_plugin_output. A summary is given in the standard plugin output.

Normally the child return code with the highest severity becomes the parent (check_multi) plugin return code.
But you can influence this by specifying flexible state evaluation rules.
The configuration is very simple: a NRPE-stylish config file contains a tag for each child plugin and then the check command line.