OpenBazaar is a peer-to-peer network, which means your computer running OpenBazaar (called a node) is connecting directly to other people's computers in order to engage in trade with them. In order to improve the connection between nodes, you can forward a port on your router in order to accept incoming connections from other users.
By default OpenBazaar uses port 4001 on TCP. Forwarding that port should be all you need unless you've manually changed the ports yourself.
To forward 4001 and accept TCP connections you need to gain access to your router's interface. If you're unsure how to do this, read this guide.
In the router settings, find the port forwarding section and add your computer's local IP address and the port you want to forward (4001) and select TCP. General instructions for port forwarding can be found here. Save and apply your settings when you've finished.
You can then check that you've forwarded port 4001 by using this port check tool. Enter 4001 and it will tell you if it's forwarded or not.