A Cookie is traditionally thought of as a food treat given as positive reinforcement. In that definition, a cookie is a discrete piece of food reinforcement.
In many dog training discussions, the idea of the Cookie is a bit less discrete and encompasses more types of reinforcers than food. The term Cookie is often verbal or metaphorical shorthand for dog trainers for reinforcement or a reward.
Petting, praise, biting, tugging, chasing, access; pretty much any reinforcement you can imagine can be thought of as a Cookie if it is given consequently.
Keep in mind there are benefits and drawbacks to this verbal and metaphorical shorthand.