Penalty Flag

What does Penalty Flag Mean?

This is an object that is held by all referees who indicate if there is a penalty or foul on the field. When this takes place the ref will throw this flag in the air, letting all players, coaches and referees know that a foul has taken place. The flag is a weighted yellow cloth.

It is also known as the "Yellow Flag." 

Sporting Charts explains Penalty Flag

The yellow flag (penalty flag) replaced the use of horns and whistles to signal fouls and mark where the penalties took place. The first time the NFL used the yellow flag (penalty flag) was in 1948 when the Green Bay Packers played the Boston Yanks.  However, the yellow flag (penalty flag) had been being used in college football as early as 1941. In 1965 the NFL changed penalty flag from white to yellow. The cloth is weighted with either beans or sand.  Prior to 1999 BBs were also used by some officials to weight the cloth. This practice was discontinued after an NFL referee Jeff Triplette caused an eye injury to Cleveland Browns offensive tackle Orlando Brown. The injury left Orlando Brown sitting out for three seasons.