This is a plug mount for the Amazon Echo Dot (3rd Generation). Note: if you're using this for an Australian Echo Dot, the power brick size is different. You can find the mount for the Aussie version here

To install, carefully push the cable through the hole where the plug sits from the back, push the power pack in from the pack, and wrap the cord around. Plug the cord into the echo dot, and carefully push it into its mount (I find it's best to put it in the bottom clips first, then work it in to the top).