The concept

The spread is the margin built into the exchange rate. If the dollar is at a certain value in the market and the conversion on your withdrawal uses a worse rate, that difference is the spread, and it's revenue for whoever did the converting.

It matters because it doesn't show up in the fee table. A platform can advertise a lower transaction percentage than its competitor and still deliver less money to your account, because the difference was charged on the exchange rate.

The honest comparison between platforms isn't fee against fee. It's how much lands in your account at the end of an equivalent sale, already converted.

How it shows up in practice

You have two gateways on your desk. One announces a rate and another announces a different one, slightly lower. You think the second one is cheaper until you do the math with a real sale in dollars. You find that the second platform converts the dollar at a worse rate than the market, and the silent difference cancels out the rate reduction.

Your margin is my opportunity.

Jeff Bezos Source: attributed to the author in published interviews and profiles.

When comparing platforms, forget the rate table. Process an identical sale on two platforms and see how much actually ends up in your account in converted reais. The spread hidden in the exchange rate is the truth the tables do not show.