I guess you may have some trouble with understanding this definition. So you may define as:
if is large enough, then will be as small as possible.
Let's do this problem in this way.
Then we suppose .
Imagine a which is extremely large. This makes both extremely small from . Since , can not be "extremely small". If you write for the sake of convenience, then we will have since . So we have found the contradiction.
If you understand this, then all the things such as will become technical ,trivial and easy to understand!!