It's called the hold-and-re-spin bonus. The idea is that the player is given a certain number of free spins in which to collect icons of some value, the prizes for which accumulate. More often than not, the symbols involved are so-called cash-on-reels symbols a symbol that simply displays a credit award.
Hold percentage, at its most basic definition, is the portion of money gambled that a casino keeps. That number varies significantly from casino to casino, how a machine is played, and the amount bet on a particular game.
