Decking out isn't explained but I felt it was such a rare thing that I didn't want to worry about cognitive load explaining it in the tutorial, at least for the jam release.
When you deck out, you lose 1 life instead of drawing a card. It's fairly rare because cards go from your hand back into your deck when used or discarded, I was attempting to enable 'force draw' as a way to deal damage, but it ended up being mostly unimportant. There are a few spells that can get a bit of extra damage through the mechanic in the fire school. I figured people who care enough about decking out would work out the mechanic themselves, and to most players it's an edge case that wouldn't occur.