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Basically, 12 hours before making your loaf you will mix together 34gr starter and 87 gr water 87 flour which you will incorporate in the recipe as your "starter". I'm not expert but I would say this is to avoid the stale starter problem which can happen and ensure you're always adding an active/fresh starter to your mix.
This is from the Rye Sourdough recipe Brickhouse used to make.
FWIW I keep mine in the fridge and refresh it once a week same time as I make my levain for my weekly loaf and never had any issues