The D5100 + AF-S DX NIKKOR 18-105mm f/3.5-5.6G ED VR supports a maximum video resolution of 1920 x 1080 pixels. That's over six times the video resolution of a 640 x 480 pixels digital camera, which is the most common video resolution supported by digital cameras that have been on the market for more than two years. In addition, this camera support a video resolutions of 640 x 424 pixels and 1280 x 720 pixels. A video resolution of 1920 x 1080 pixels is equivalent to Full HD video. You will require a Full HD TV or better to appreciate this level of detail.
A 1920 x 1080 pixels video resolutions is in fact the best supported video resolution amongst all medium megapixel cameras that are a comparable age to this digital camera.
Amongst comparable types of camera the worst supported video resolution is 320 x 120 pixels.
What does "video resolutions" mean?