How does big O notation indicate upper bound on a function?

Suppose we have a function, f(n)=n+10. We say that f(n)∈O(n). Meaning O(n) represents the upper bound of the function.

Its is also true that f(n)∈O(n^2) or f(n)∈O(n^3) and so on.

So how can we say that O(n) is the upper bound when O(n^2) is comparatively bigger bound and O(n^3) is much bigger bound?

Please forgive if I have understood the concept wrongly.