Best buy is the highest price level that someone puts in to buy the stock, that hasn't been matched.

Best sell is the lowest price someone puts in to sell the stock, that hasn't been matched.

So, if you want buy the stock, you can put the order at the best sell and it will get matched. Trade done.

Or, you can put buy order at lower price. Here, no trade is done yet, you wait for someone to hit you/sell to you. N If your buy order is the highest price, then it will become the best buy price. N you wait.