I don't quite understand this....hmmmm

Once upon a time a very strong woodcutter ask for a job in a timber merchant, and he got it. The pay was really good and so were the work conditions. For that reason, the woodcutter was determined to do his best.
His boss gave him an axe and showed him the area where he was supposed to work.
The first day, the woodcutter brought 18 trees.
"Congratulations," the boss said. "Go on that way!"
Very motivated by the boss' words, the woodcutter tried harder the next day, but he only could bring 15 trees. The third day he tried even harder, but he only could bring 10 trees. Day after day he was bringing less and less trees.
"I must be losing my strength", the woodcutter thought. He went to the boss and apologised, saying that he could not understand what was going on.
"When was the last time you sharpened your axe?" the boss asked.
"Sharpen? I had no time to sharpen my axe. I have been very busy cutting trees..."

by: Stephen Covey, 7 Habits of Highly Effective People
___________________________________________________________

So the part that I don't quite understand is... what does it mean in practical terms to 'sharpen your axe'?? Can anyone please enlighten me?