Outbound sales is the process of reaching out to prospects and making sales pitches. This is different from inbound sales, where a potential customer reaches out to you as a result of your inbound marketing efforts.
Let's think about the alternative for a second: Inbound sales typically starts with content marketing. It's a slow path to follow and leads that come through this path span a are not targeted in any way.
On the other hand, with outbound sales, you reach out to your ideal customers and directly pitch your products and services at them. This kind of outreach is highly targetted, and results in immediate feedback.
More importantly, once you figure out what kind of pitches work, you are in control of scaling the sales process.