Forgive me for being clueless, but what is OTG?
USB On-The-Go. Basically allows mobile devices to act as both hosts (I.E: Your desktop computer) and as peripherals (I.E: A flash drive). In essence, having it enabled would allow you (via an adapter, of course) to plug in USB devices to your phone. Android has supported this for many years (and so does iOS, but only to an extremely limited degree). Peripherals that can be used (assuming the OS and/or app(s) being utilized support it) includes, but certainly isn't limited too: Mice, keyboards, game controllers, head-sets, flash drives, cameras, etc.
So in essence, you phone can become a host for using such peripherals On-The-Go (hence the name), but still then be able to switch back to being a peripheral once connected to another host (I.E: your desktop or laptop).
Very useful, hope they add support with future hardware and/or software updates.