There are many types of barlines, which are simply vertical lines that go from the top to the bottom of a staff. They usually signify a division between sections.
A
single barline is the division between measures of music.
A double barline
represents a major division in the music, such as a change in tempo, the
start of a repeated section, etc.
This
type of double barline represents the start or end of a section
of music.