Rebuilding Thumbnail Cache?

[stork]

What does it mean when it asks to rebuild thumbnail cache?
When i use iPhoto for a while, e.g a couple of hours.
Ive imported alot of photos recently due to only have bought the macbook a few days ago, and have been keywording and re-naming.
Does it damage and is it normal?

 

[Zwhaler]

I'm not sure what it does but I've done it on all my photoa (four and a half thousand) and I heard its a good idea and I don't see why it would exist unless it was a good thing. You can still use your computer while it is doing it.