what is a variable ratio schedule

A reinforcer is given after a specified number of correct responses. This schedule is used to increase and maintain a steady rate of specific responses. This schedule is best for maintaining a behavior.

A high steady rate until reinforced
No (or only a very small) pos-treinforcement pause.
The value of a variable rate schedule can be increased a greater rate than a fixed rate schedule without producing ratio strain, and
A high RTE and the values of variable ratio that can maintain responding are somewhat higher than those of a fixed ratio.

Examples in natural environments:

Slot machines. Slot machines are programmed on VR schedule. The gambler has no way of predicting how many times he must put a coin in the slot and pull the lever to hit a payoff but the more times a coin is inserted the greater the chance of a payout. People who play slot machines are often reluctant to leave them, especially when they have had a large number of unreinforced responses. They are concerned that someone else will win the moment they leave.
Playing golf. It only takes a few good shots to encourage the player to keep playing or play again. The player is uncertain how good each shot will be, but the more often they play, the more likely they are to get a good shot.