ROAS (Return on Ad Spend) is the revenue generated by ads divided by the amount spent on those ads. For example, a campaign that generates 4,000 euros in revenue from 1,000 euros of spend has a ROAS of 4 (or 4:1).
ROAS measures revenue efficiency, not profit. A ROAS that looks healthy can hide a loss if margins are thin, returns are frequent or shipping and payment costs are high. That is why it helps to work out break-even ROAS: the minimum level at which a campaign covers the cost of goods and the ad spend itself. In simple terms it is 1 divided by gross margin expressed as a fraction; with a margin of 0.4, break-even ROAS would be 2.5.
Attribution is the other thing to watch. Each ad platform tends to credit itself with the sales it touched, so the sum of ROAS reported by several platforms can exceed actual revenue. Comparing against store data and against MER (total revenue divided by total marketing spend) gives a more realistic picture.