Here is how Nunu(and most other control jungler) works
does the following, ranked from THE MOST IMPORTANT to the LEAST IMPORTANT
- Control vision in enemy jungle, so enemy jungler can't gank as easily since they will be tracked.
- Steal enemy jungler's camps, so enemy jungler become useless due to starvation.
- Enable teammates (imagine your
with 
on no cd ) and disable enemy (Perma slow) - Gank
This is a very complicated process and requires a lot of experience, but you don't need to know any of the above. just look at what you need to do below.
So for to do things and not be useless, you need to:
- Look at the mini map, so you don't feed the enemy Jungler who was starved. http://www.mememaker.net/static/images/memes/4063433.jpg
