Bar numbers are found at the start of each line of music. They are used extensively in Playing Guides to allow you to find the specific bar I'm talking about easily. When playing in a band of 'reading' musicians they are used in the same way to make sure you are all discussing the same area of the piece.
When a repeat mark is used, the bar number following it includes the bars played in the repeat. The same applies with D.S. and D.C. al coda markings. This means that if you are playing with another musician who is using a transcription for their instrument, your bar numbers should match up.
Where a song starts with feedback or other noises, the bar numbers may start where the actual music starts. You will notice in songs where this applies, bar 1 appears later on in the score.
