Hi Pradeep,
In general, Inbound offers are the offers which are not sent to the customers wherein the customer visits the offer on its own. For example, Webforms/Surveys where a customer reaches by browsing a website.
Outbound offers are the offers which are sent to the customer through any of the channels. Example, Emails, SMS etc.
You may refer the link: Offers on an inbound channel
Thanks,
Prateek Malik