Free Play is an unofficial / “slang” term for a delayed penalty.
It’s called a “free” play because, per penalty possession rules, once Team B (the offender) gains possession of the ball, the play will immediately be blown dead and Team A (the victim) will be given possession again.
Thus the a turnover is impossible during Team A’s possession. This lowers Team A’s risk, and may make Team A more likely to attempt an aggressive or risky play.
“Sure, run a crazy play! Why not? Even if it doesn’t work, you’re just going to get the ball back again anyway.”