Let me set the scene.
I have a friend named Matt.
Like I said, Matt is a great photographer (and videographer, but we won't get into that).
Basically, my new 35mm F1.8 is amazing. I love it, but I have been having issues with shooting in low light. When you shoot in low light you need to have a longer shutter to let in more light, or open up your aperture (depth of field). If you don't, your picture will be too dark. Think of your aperture as the iris of your eye. It shrinks when its bright to reduce the amount of light that is let in, and opens wide in the dark to let in more.
On cameras like my D5000 there are many different settings. You can have the camera do all the thinking and set both the shutter and aperture based on what its light meters read, or you can do everything on your own. I usually either set one or the other. Meaning, if I was shooting something in motion, I would set the shutter speed, and let the camera decide what the best aperture was. If I was more concerned with what was being captured in focus, I would play with the aperture and let the camera decide the shutter speed. Never, under no circumstances, did I ever control both. Terrified of that. Until Matt encouraged me to try it out....
All of these pictures are straight out of my camera:
The photo above was shot with me controlling the aperture which was set at 3.5, and the camera set it to shutter speed 1/25. (ISO was 200 for those interested). Obviously, this photo needs more light, but because of the window on the left, the cameras sensors were off. I could lower the aperture to let in more light, but that would compromise the depth of field. I already have the aperture pretty low, any lower and I might lose too much of the picture. So I took control and set both.
Thanks for the tips Matt!