There are two ways to pay in OpenBazaar.
1. Direct Payment - The buyer sends the bitcoin directly to the vendor.
2. Moderated Payment - The buyer sends the bitcoin into an escrow account and they are released when the transaction is finished.
Moderated payments give the buyer some protection against the vendor taking the money and not delivering the product. But it's important to choose a trustworthy moderator. You can read about choosing a good moderator here.
If you choose moderated payments, then your bitcoins will be sent into an escrow account (using a unique feature of Bitcoin called multisignature addresses) that will only allow the funds to move if two out of three parties agree. Those parties are the buyer, vendor, and the moderator. This is more secure than a direct payment, in which the vendor receives payment as soon as they process the order.
Not all vendors offer moderated payments. If they don't, you won't see any moderators to choose from on the order. For vendors you trust, or small orders, a direct payment works just fine.