In digital photography, what does it mean to expose for?

I'm a noob when it comes to digital photography and I have been hearing this a lot, eg. "we'll expose for the shadows first and then for the bright portions… " etc.how's this accomplished? What is the purpose? I have a Nikon D90 (fyi)… Any examples or explaination you guys can share?

Keep the shutter open for a set time.

Expose for the shadows and develop for the highlights is what photographers who shoot black and white sheet film do to control the gamma of the negative

When shooting multiple images used to merge when making HDR images, it is important to make the OE image set to expose for the shadows, UE set to expose for the highlights and of course a normal exposure. They are then merged to to provide an image with detail in both the shadow and highlights

It means that just before you click the shutter, you should drop your pants… It doesn't really help the photo and on cold days it can be a little injurious to the male ego but it's a great way to get back to nature.

In film or digital, in every "picture" there's a particular place, area or 'something' YOU as the photographer want to have in perfect exposure. Most cameras take a over all average of a scene and end up having some areas way over or under exposed. If the 'thing' is in the shadows, expose there… If in the brighter light, expose there. If a baby fox is under the bushes where it is dark, you sure don't want the camera exposing for all the bright sun that could be around it, right? The baby and shadows go black. And by the same token, the Red Winged Blackbird on the sun lit tree branch, you don't want to over expose it by exposing for a lot of the dark tree… And the blue sky goes white and the colors on the bird all washed out.
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One of the ticks is to expose for the the main subject, large or small, regardless of what it is and not for the whole scene.
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Where is the light coming from, what direction. Where is it falling on your 'item' . What actually is being lit up and why do you like it enuf to take a picture of it. Your minds eye is seeing the view in a certain way, now, just how do you capture it in a camera the same way your mind saw it?
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Most just whip the camera up to their face, and shoot! Look later and say, aww, dang it, but you kinda see what I was trying to get, see?
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Most leave their cameras on P (full Program or Automatic) and wonder why the pictures keep escaping them. Most don't know how light meters work, what 18% gray is, spot metering or average, or what bracketing means. Most don't wanna know. Too much trouble. Too much to learn.
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Your D-90 is capable of some remarkable stuff if YOU understand it and take it off that Gawd awful green P and learn the M and how your light meter, shutter speeds and f-stops work. You also have a spot meter in the camera. Get out your instruction book and read the dang thing. When ya set on the pot, take the book in hand, not your self or a Playboy and read the stupid camera's manual.
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Expose for the part of the vision that caught your eye in the 1st place. Not for the whole scene. Many sunsets are ruined because of this and most end up being all washed out. You need to expose for the brightest part of the sunset and actually under-expose things. Make it darker.
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The moon in a dark sky? Most have a gray sky and a super over exposed blob of white as the moon. Learn why! Learn what your light meter is trying to do, and does, to screw up your attempted images.
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Some one on a stage by spotlights. Same way. Nice colors seen by the eye but to the camera, all washed out and pale… Again, light meter lied and P screwed you again!
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Good pictures are a combo of a couple things. Composition, subject matter and exposure. All 3 have to be on to get it and make a picture worth it.
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