What it looks like
An interdental brush looks a bit like a tiny pipe cleaner attached to a plastic handle. The brush itself is only about a centimetre in length, and can come in a variety of thicknesses. The wire of the brush is usually between about 0.4mm and 1.3mm in diameter, depending on the brand's range. Lots of tiny, flexible bristles poke out of the wire in all directions perpendicular to it. These are the bits that actually do the gentle scraping of unwanted debris from your teeth.
What it does
It is designed to help you easily remove bits of food and plaque from the gaps between your teeth, just below where two teeth touch each other, next to the gum. The hundreds of tiny bristles attached to the main wire make it much more effective at this task than dental floss, although dental floss is thinner so it can reach between teeth that are overlapping.
How to use it
First, bend the wire so that it is at an angle to the handle. Then gently push the wire between your teeth. Never force it, as this can be bad for both you and the brush. Usually after changing its angle ever so slightly a few times, you can stumble upon the correct angle and it will slide right between your teeth. Just gently slide it back and forth a few times, then pull it back out. It shouldn't hurt, although you may bleed slightly if you have gum disease. Repeat the process for all the gaps which the brush fits in. Use regular dental floss on the rest.
Once finished, clean the brush carefully with running water, then bend the wire back to how it was before.