Hybrid is currently a big hype. Where a year ago the cloud was the way to go and on-premises advised against, companies have now realized the cloud is not suitable for every situation. Especially government organizations or big companies are not able to migrate everything to the cloud. So, Microsoft has invested a lot in hybrid to bring the best of two worlds together: the flexibility and data sovereignty of on-premises and all the cool new features of the cloud.
Part of this is Cloud Search: Make your on-premises documents available in cloud searches, i.e. your SharePoint On-Prem documents should be visible in a search in Office 365. Until September 2015 the hybrid search was meh, it meant two different result sources / verticals. Introduced in August 2015 is the real hybrid search: search results from both on prem as well as Office 365 are combined.
I have set this up and was surprised how easy it was. As a SharePoint guru I’m used to errors, but I haven’t encountered ONE (due to software faults, errors encountered were solely my fault). And that’s pretty amazing as I’ve used SharePoint 2016 which is currently in public preview.