Jetrim got it.
When you change aperture you need to change shutter speed, most of the time anyway and the green dot tells you about focus. I've heard of the KatzEye but haven't used it though I did consider it because I really miss the split prism focus aid that was on older lenses.
Lower light = larger aperture or longer shutter or higher iso or any combination thereof.
A tripod isn't usually necessary at 1/60th and above but below that you better start considering it unless you have a vr lens.