The phrase "interchangeable backs" refers to the ability to switch or swap different parts or components of something, usually referring to the back portion. Full definition
The design and looks are very similar to the original Moto G, and comes with interchangeable back covers sold as accessories. (techpp.com)
It also has colorful interchangeable back plates, which is nice. (theverge.com)
The company offers a ton of colorful interchangeable backs across four different styles so you can swap out the rear cover plate as often as you change your clothes or hairstyle. (tomsguide.com)