Coinbase acts as an exchange. People who have bitcoins and want USD, along with people who have USD and want bitcoins use coinbase to facilitate their transactions.

Lets say I want to buy bitcoins for 600/coin, so I post an "bid order", that tells people I'm willing to give them 600usd for a bitcoin.

A merchant has a bitcoin that he wants to turn into USD. He goes on coinbase, sees that I'm looking to buy a bitcoin for 600usd. He decides that he can do better, so he creates an "ask order", that tells people that he is willing to sell a bitcoin for $650usd.

On coinbase, thousands of these bid and ask orders are on the same exchange. When you see a buy price of $809.58, that means the absolute lowest ask order is selling some amount of bitcoins for $809.58 each. When you see a sell price of $786.10, that means that the absolute highest bid order is buying some amount of bitcoins for $786.10 each.

If you were to go on coinbase right then and buy a bitcoin for $809.58, the absolute best price you could sell it at is $786.10 because that's what the highest bid order is offering. If you're wondering where the extra $30 went, it goes to the guy who you originally bought the bitcoin from.