Plugins work by leveraging the API of the BI tool you are trying to connect to. To be able to connect to the BI Tool, certain parameters must be provided by the user and defined in Metric Insights. This includes the hostname, server URL, username, password, etc. Note, the parameters can vary based on the BI tool you are trying to connect to. 

Once those parameters are defined in Metric Insights (stored in a database table), Metric Insights parses the provided information into a file for the plugin to read. The plugin then takes the provided information to connect to the BI tool, and once authenticated, executes the specified job (getImage, getData, etc.).

To see the list of available plugins for Metric Insights, please go to http://help.metricinsights.com/m/Connecting_to_Data_Sources/l/104450-metric-insights-plugins.