The idea of this AVnav plugin is to display an instrument that contains all the basic information needed for sailing. With the possibility of showing this display directly on the map at the boat position, the sailor has all the information at a glance. The laylines give you the quickest bearing to a waypoint upwind, and when displayed on the chart you can follow those lines.
If you want to know what laylines are, you can find some information here:
Fig: Sail Instrument Plugin for AV nav
The inspiration for the instrument is based on B&G's Sailsteer instrument. There is a project on GitHub for the Sail Instrument where you can find all the important information about the plugin.