Measures are groupings of notes, usually grouped by meter. Bar Lines show us where one measure ends and another begins.
There are different types of Bar Lines, each with a different function
A Single Bar Line is the normal Bar Line, it tells us where the measures are:
A Double Bar Line shows there is there is a major change in the music, such as a new musical section or new time signature:
A "Final" Double Bar Line shows that the piece of music is over:
