Is a simple and easy overview of the number of shares / instruments that are for sale or that you want to buy. Sellers and buyers have then submitted an order on an exchange with a desired sale or purchase price for a specific instrument. These orders are then placed in an ordered list called the order book. The number of shares for sale is called ("Seller") and the number of shares that are desired to be bought is called ("Buyer"), they are collectively called order depth.
The order book is divided into different levels. The best price in relation to the last traded price is at the top, the second best is in second place and so on. Most stocks / instruments also have a minimum permitted number of decimals that are allowed on an order. This varies depending on the instrument in question.