The middle line is the equator, the second and fourth line are the tropics, the first and fifth are the polar circles.
What is the relevance of these bands on Earth? Do these exist on other planets? What purpose do they serve?
The Tropics (Capricorn in the South and Cancer in the North) are the furthest point from the equator where the sun reaches a point directly overhead at any time during the year. The Arctic and Antarctic circles are the points at which the sun remains above the horizon for a full rotation of the Earth (i.e. 24 hours), commonly called "Midnight Sun". Any planet that has axial tilt while have similar lines.