What is an instance of an object?

A typical "example".

For instance, the computer your are using to view this answer is just an instance of (the class Personal Computer), there maybe an "identical" setup (software and hardware components) out there as yours, yet, the "identification tag" (in some object-oriented languages it's called Object Identity) are different.

Another way to look at this: Class provides the abstraction, the instance of that class provides the realization of that abstraction. A human is an abstraction, and you are a realization of a human (an instance of human, but not the human class itself), a typical example of a human.