A Bar (referred to as a Measure in American English) is a small segment of time that contains a specific number of beats as defined by the time signature.

1. Bar Lines: In sheet music, these are the vertical lines that separate one bar from the next.
2. Purpose: Bars break down a piece of music into manageable "containers," making it easier for musicians to read and keep track of their place in a composition.

For example: | 1 2 3 4 | 1 2 3 4 | …Each bar contains 4 beats.