Outreach is simply the process of reaching out to companies, organizations, communities, or individuals with the intention of building a relationship that is mutually beneficial and educational.
The primary difference between Outreach and, say, Public Relations is that Outreach focuses more on groups that may not otherwise have had access or been aware of the services you provide, whereas Public Relations efforts often start with the premise that your service or company is known.
A striking example of this might be that of entering a new developing market somewhere in the world. For instance, if you were to bring the idea of an education startup incubator to a foreign country that has never seen one, you would most likely start with reaching out (read: outreach) to the communities and government about what you were bringing to their country.
In this sense, Outreach is much like a pre-cursor to PR – you are reaching out to a group with the intention of building a relationship and familiarity with what you do.