Music is written on a staff, which consists of five parallel lines between which there are four spaces.
All of the individual parts of the drum kit are written on different lines or spaces.
Bar Lines are drawn across the staff, to divide the music into sections called bars or measures. Bars are used to break the music into smaller digestible parts, much like spaces between words.
A double bar line signifies either the end of the music, or the end of an important section of it.