His message behind this quote is that preparation is a key ingredient to success. You would spend 4 hours sharperning the axe, allowing you to get a sharp axe. This means you can chop the trees within a short period of time. So always prepare for any task beforehand.

Preparation. Having the right tools for job makes the task easier and makes you more effectual and efficient. If the axe is sharp - i.e. the tools prepared - the cutting down will take less time. 

Hi!

It means that prepartion is good in order to do something proper.