On: The story behind football's innovative yellow firs...

[ken_railey]

It represents the "line of scrimmage", which is where the ball starts at the beginning of each play.

[chrishas35 → ken_railey]

The yellow lines marks the line to gain for a first down, not the line of scrimmage. Some broadcasts are showing the line of scrimmage now usually as red or blue IIRC, but yellow is used across the board for the line to gain.