Let’s say you’re a gas station. You need to spend 500k to get things running including buying 100k of gas. Every week rent and people’s salaries add up to 19k and you sell gas costing you 80k for 100k.

So, after 1 week you have 20k in gas and 100 - 19 = 81k in cash. You need to spend 80k of that on more gas for text week resulting in a net profit of 1k or a 1% profit margin.

But, repeat that for one year and you bought 80k x 52 weeks of gas = 4.16 million even though you only had 500k in capital. Further your profit margin might be low but your ROI was 52/500 or 10.4% which is not bad.