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.
In gambling, the hold percentage refers to the portion of money that the casino keeps from the total amount wagered by players. It is also known as the "hold" or "win" percentage, and it represents the casino's profit from the bets placed.
