# How does f(n) < cg(n) specify time?

I have been reading this tutorial on time complexity, and I am a bit puzzled on its explanation of big $$O$$ notation. It writes:

$$O(g(n)) =$$ { $$f(n)$$ : there exist positive constants $$c$$ and $$n_0$$ such that $$0 \leq f(n) \leq c*g(n)$$ for all $$n > n_0$$.

The way I understand it, $$c*g(n)$$ denotes $$c$$ multiplied by $$g(n)$$. If this is the case, then how does the above expression specify time, and not just that $$c*g(n)$$ is greater than $$f(n)$$? Or am I incorrect, and this is some notation I don’t understand? My understanding of time complexity is very rudimentary, so forgive me if this is just a stupid mistake on my part.