Safety chains are located on the tongue of a trailer, one on each side. The end of each chain features an S-hook, which connects to a hole in the hitch. Should the coupler or hitch ball detach from the tow vehicle, the chains keep the trailer connected. Most trailer owners cross the chains under the trailer tongue to help prevent the tongue from dropping to the road in such an emergency. They also make sure there's enough slack in the chains to permit turning, but not so much slack that the chains drag on the road surface.
You should secure safety chains properly every time you tow. In fact, most states require that you do so. Most states also require that safety chains and hooks be rated to handle the gross trailer weight. Check with your local motor vehicle administration to find out what requirements affect you.
Next, we'll talk about a hitch accessory that you can customize and have some fun with.