IPhone 5s will not back up iCloud automatically or manually?

I got a message saying my phone hasn't been backed up to iCloud blah blah, and that in order to do so it needs to be locked, connected to Wi-Fi, and plugged into a power source. What exactly do they mean by power source? What do I do from there? Also, I've tried going in my settings and hit the "back up now" button to do it manually, but each time it's gotten and hour in and tells me "back up can't be completed". Can someone please tell me what's going on and how to fix it?

It means plug it into a charger. Maybe your internet had a spas and went offline for a second? Also try to goto an Apple store if it keeps giving you trouble