This is the idea the whole site is built around. Once it clicks, every other number makes sense.
An order book is not one price. It is a stack of them, each with a limited number of items behind it.
$500 1,000 units $490 20,000 units $450 500,000 units
The best price is $500. But only a thousand items can be sold at $500. After that, the next buyer pays $490 — and once those twenty thousand are gone, $450.
The mistake
1,000,000 × $500 = $500,000,000
This is fiction. There are not a million items at $500 — there are a thousand.
What this site does instead
It walks down the book level by level, taking what is really there at each price, and stops when the next unit would no longer be worth selling. The total it arrives at is executable profit: what the market as it stands can actually support.
That is also why a trade can have a spectacular price and a small executable profit. The price is real; there is just almost none of it.
Reading it on an item page
The standing-orders panel on any item shows those levels. Units at the top is how many are queued at the best price — the ones you would be stepping in front of. Price levels is how many distinct prices exist at all: one is a single buyer, a dozen is a market.