That is the EE servo coupling switch. This is only for use with older lenses (non-G AF lenses). Modern AF-S G lenses don't have an aperture control ring (it's completely controlled by the camera body).

When a non-G AF lens is mounted, the tab on the lens engages the switch on the camera when the lens's aperture ring is stopped down to is minimum aperture setting (such as ƒ/22 or ƒ/32, depending on the lens). This is lets the camera know that it has full control range of the lens's aperture. Any setting of the lens aperture ring wider than minimum means the camera cannot physically stop the lens down lower (smaller) than the number on the aperture ring. For instance, if the aperture ring were set to the widest aperture setting (say, ƒ/1.4 on the AF 50mm 1:1.4D), then the camera would not be able to change the lens's aperture at all.

If you mount a non-G AF lens to your D3100, the camera will tell you when the aperture isn't stopped all the way down: it will give you a fEE error, and won't let you shoot until you stop the lens aperture ring all the way down. See: Why is my Nikon 50mm f/1.8 giving me an fEE error?

See also:

• Christoph Breitkopf's Nikon AI Conversion — hover over the first images to see the aperture ring engage the coupling switch.
• Using Older Lenses on Your Nikon DSLR (throughthefmount.com)