Break-even ROAS shows the ad efficiency needed for an order to reach zero profit. If the required ROAS is too high for your channel, the offer may need a higher price, lower costs, or a different acquisition strategy.
The simplest version is:
Revenue divided by profit before ads.
If an order has $100 in revenue and $40 in profit before ads, the break-even ROAS is 2.5x. Spending more than $40 to get that order would make the order unprofitable before considering lifetime value.