Time Complexity

In this:
\mathcal{O}(n^2 + m), because it consists of two blocks of complexity
\mathcal{O}(n^2) and \mathcal{O}(m), and neither of them is a lower order function with respect to the other.

What does a lower order function mean?

Thank you.

For example, \mathcal{O}(n^2+n) can be written as \mathcal{O}(n^2) because \mathcal{O(n)} is a lower order term. In this case, it’s hard to say that \mathcal{O(m)} or \mathcal{O}(n^2) necessarily grow slower compared to the other.