Adobe Photoshop CS 2 User Requires Help!!

Hey everyone... I'm trying to follow a Tutorial on How to make a planet by Greg Martin... And I've gone through just about 80% of this Tutorial without problem...

But this Tutorial is designed for Adobe 7 - so some of the things he talks about are moved around within the software... Can someone help me figure out what the following steps are referring too? :

"We need to isolate the light and dark elements of the embossed layers, because we'll play with both seperately (hence the two layers.)

Use levels on the topmost one to isolate the lights by increasing the darkness until the main part of the layer is black with just a few white squiggles. This is now our "highlights" layer. You might want to rename it as such."

"Repeat this with the lower embossed layer, only this time pull the lightness up so you've got all white with black squiggles. This is now our "shadows" layer. Again, it might be a good idea to rename it as such."

([link])

I'm not entirely sure what he means when he says "Use levels on the topmost one [layer]"

What exactly is this "Levels" tool hes talking about, and if someone knows - where do I find it?

Thanks for reading.