Each star stands for one year of being a captain on that particular team. So if they have 2 stars, they have been a team captain for that team for 2 years, last year and this year.
The initiative has been in effect since 2007, the most gold stars that a captain can have right now is 3. Drew Brees & Carson Palmer are examples.
It's not clear why the NFL picked 4 stars-
Perhaps it's because they fit easily.
But a gold star is given for every year that the player is a captain. I do not know what will happen if someone is a captain for more than 4 years. Perhaps the color will change. In any case, as this is the first year of the patch, no one has more than one gold star.
According to Paul Lukas in an article in ESPN Uni Watch from 2007:
....team captains this year will be wearing a specialcaptain's patch on the upper-right chest area (as you can sort of seehere). The patches will be color-coordinated to match each jersey, and the gold star underneath the "C" marks the number of years that the player has been a captain under the league's new captaincy initiative.
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The stars on team captains jersey's (Peyton is a team captain) are white, but the first couple stars are usually yellow. This represents how long they've been captain.
Each star represents a year of serving as a captain. So 4 stars represent serving 4 years or more in the same team