Wild
Triple Seven Wild
The Triple Seven lands on all three reels and replaces other symbols on a payline. A line made only of Wilds still pays. During Free Spins the same symbol does extra work: every Wild that shows also drops a direct cash prize on top of line wins.
One detail that catches people out: three Triple Sevens on a line do not start Free Spins. Free Spins need their own scatter-style symbols. Wilds stay Wilds.