I've posted this before, but here it is again. Hope it helps!
***
This would be easier with pictures, but I'll give it a shot without.
So the histogram looks like a mountain range in a box, right?
Really, it's a map of the number of pixels across your tonal range. The
far left is pure black, and the far right is pure white, and the middle
is everything in between. It doesn't matter how many peaks you have or
how high; what you're concerned with is where your mountain range
starts and stops inside the box.
Ideally, it starts at or right next to the left and ends just before
or at the right wall. That means you have the full tonal range in your
image.
If it's all lumped up over to the left and you have empty space to
the right, you're underexposed.
If it's "climbing up" the right wall, you're overexposed. You've
"clipped" your highlights, meaning that you have some details that are
all going to show up as pure white. This will happen if you have
spectral highlights, like chrome or water, or a light source and that's
ok. Or you may want to expose for your subject and let the background
overexpose if your subject is backlit. But you don't want your skin
tones clipping. If the red channel clips in your skin tones, that means
that where there would be very light red, there's just white, and it's
not pretty. If you shoot in RAW, you might be able to bring back those
highlights, but the goal is not to need to.
If it's "climbing up" the left wall, you've "clipped" your shadows.
Instead of detail, you're going to have pure black. Incidentally, when
you "increase the blacks," you're moving that left wall closer in to
your mountain range. If you move it just to the edge, you're giving
yourself a full tonal range -- now your image goes all the way to black
instead of stopping somewhere short. If you move it under a peak, you're
starting to lose shadow detail. Again, you may decide in certain
circumstances that you want that, and that's ok.
When I check the histogram, I'm looking to see how close my mountain
range is to that white wall, especially the red channel because that's
the skin. I want it to be pretty darn close, but I also like to leave
just a little bit of wiggle room so I don't accidentally overexpose.
A lot of times if I meter at 0, there's a lot of room to the right.
It may be technically right, but I like my images brighter than that,
and brightening them in post can introduce ["exaggerate" is a better word] noise, so it's best to get
them as bright as possible without clipping my highlights in camera.