A retrigger happens when the symbols that started a free spins round land again during that round, adding more spins. In many games three scatters during free spins add the same number of spins again; in others a smaller number of scatters adds a few extra.
Not every game allows retriggers, and some cap the total number of free spins. The paytable explains the rule. Some games use a retrigger to move you to a higher level of the feature, with a bigger multiplier or more wilds.
Retriggers are part of the feature's expected value, so they are already counted in the published RTP. They are one reason feature results vary so widely: most rounds end without one, while a few run long.