No-arbitrage bounds

No-arbitrage bounds are a set of rules that help people decide how much they should pay for something. To understand them, it's helpful to think of a game.
Suppose two friends are playing a game and they need to decide how many points each person will get. They can agree on any number they want, but a no-arbitrage bound says that the number they choose must be fair.
In other words, one friend can't give themselves all the points and the other none. That wouldn't be fair. Instead, the two friends have to agree on a number that is between the two of them - a number that is fair and doesn't give one friend too much of an advantage.
A no-arbitrage bound is similar for buying something. Instead of points, it helps people decide what price is fair for a product or service. The no-arbitrage bound say that the price can't be set too low or too high. It should be somewhere in between the lowest and highest price that it could be set at. This way, no one is getting an unfair advantage.