"Package any app for every Linux desktop, server, cloud or device, and deliver updates directly." - http://snapcraft.io/
A snap is a fancy zip file containing an application together with its dependencies, and a description of how it should safely be run on your system, especially the different ways it should talk to other software.
Most importantly snaps are designed to be secure, sandboxed, containerised applications isolated from the underlying system and from other applications. Snaps allow the safe installation of apps from any vendor on mission critical devices and desktops.