"Come on, faster!"
It literally means "add oil". If the student is from an English-speaking background, I often translate it as "put some elbow grease into it". It's an old expression, but sometimes you hear it.
加油 means to add oil or to refuel. For example 加油站 would mean gas station. Individually the characters are 加 add, and 油 oil.
as the other's have said it means "add oil", but oil as in petroleum i.e. gasoline/gas that goes in a gas/petrol tank, equivalent English expressions in BrE and AmE would be to "put your foot down on the accelerator" and to "step on the gas"