I find the terminology a bit confusing too. If I understand it correct, you can think of "borrowing" in this case as creating or issuing new bitUSD tokens. You would have to back these tokens with your own funds as collateral. You are "borrowing" a newly issued bitUSD from "the network" in the sense that 1) You can return that bitUSD to "the network" at any time to get your collateral deposit back 2) "The network" can reclaim its bitUSD back at any time (i.e. when your collateral becomes less than a threshold or when some sort of forced settlement is required) by buying bitUSD on the market on your behalf using your collateral funds, keeping (and supposedly destroying) the purchased bitUSD, and handing over the remaining collateral to you.