Past Life: Disciple of the Fist.. what are the dice step increases ?

[humbleroller]

So i understand that monks start out with a 1d6 and then progress on to 3d6 by level 20.

I have 2 past monk lives and I was wondering what the past life feat Disciple of the Fist really does.

...deal increased unarmed damage (one step higher than normal on the unarmed combat chart)

what does 1 step higher mean ?
how many times can I take this feat ?

-thanks.

also, is Improved Critical: Bludgeoning worth it? Does it make handwraps go from critting on 20, to 19-20 ?