"pre-rolling" is where you bet a small (or zero) amount over and over until you lose N times in a row before starting your Martingale sequence in the mistaken belief that the N losing rolls uses up most of your losing streak such that you won't lose too many more times before the streak ends.

Of course in reality the only way the pre-rolling actually helps is that it reduces your average bet size and so reduces the rate at which you lose money.

I guess the spreadsheet is calculating the expected number of pre-rolls required to generate N losses in a row, calculating the expected cost of making all those bets, and adding that loss to the various possible profits and losses from your Martingale streak.