This is an order you can specify with your brokerage to create a rules to protect you from entering positions at prices you weren’t expecting. This is important for illiquid stocks.

If you want to buy XYZ trading at $20/share, buying a lot might mean that you fulfill all the sell orders at $20 and start buying from those trying to sell at $21. Placing a buy-limit at $20 would tell your brokerage that you won’t pay for XYZ for anything over $20/share and your buying would be put on pause until you can buy more shares at $20/share.