the valley is the low spot in the draw cycle, how "wide" it is depends on the cam, generally speaking the faster bows have a very small/narrow valley. As you draw the bow, the force required to draw back climbs up to the peak weight, then stays there for a bit, then drops off into the valley where it gets easier to pull back (this is when the cams roll-over). The valley is that spot where it's very easy to hold back. A bow with a wide valley (hard to find now) generally will feel "soft" there, meaning you can actually pull further back and let up a little without the string trying to jerk your arm off. Most bows now have a narrow valley and a firm backwall, so as you drop off in weight, you quickly hit a "wall" where the bow won't draw back further. This is shooting off the wall and is how most bows should be shot nowadays.

You should ALWAYS get into the valley, it feels like the string just got alot lighter to pull back and hold. This is where let-off is calculated, or holding weight.

Yes, that's the valley, sounds like a narrow one and a firm wall. It might mean your draw length is a bit to long, but hard to tell without seeing a picture. With a narrow valley if you letup at all, its going to want to go all the way, letting down takes some getting use to here as well. You SHOULD relax at full draw, as much as possible.