Could you explain the module legend to me?

In the main menu, units become colour-coded as they are studied along the module. Units initally appear in light grey. Once a unit is started and in progress, it is signaled in blue. Light colours appear once units are completed.
Inside each unit (in the unit view) and each category (in the category view), grey circle (or the words “Not Started”) indicates that a section has not yet been approached.
Blue circle (or the words “In Progress”) indicates that a section has been started but its requirements have not yet been fulfilled.
Green circle (or the word “Completed”) indicates that the requirements of a section have been fulfilled.