When you get a call, Hello shows you information about who’s calling based on his/her Facebook profile. Only shared info is shown, but you may see a name and profile image, number of mutual friends you share, and other public tidbits to help you decide whether to pick up or not.
And for calls you don’t want to pick up, Hello gives you more control there. Numbers that are commonly blocked by Hello users will be highlighted as such, and you can even set the app to automatically reject calls from heavily-blocked numbers. In those cases, they go straight to voicemail, and you can decide then whether or not to properly block the number yourself, as well.
Hello is also useful for looking up businesses and establishments, pulling info like hours, location, and reviews, and then you can dial directly from the app - with free calls available over Messenger. In other words, Hello is another way for Facebook to get people into its Messenger platform, but it also serves a purpose for standard calls, as well.
Facebook’s Hello app is available now for free from the Play Store, although the company says it’s still testing the app - so things could change. Seems worth a look, at least, assuming you’re not too paranoid about Facebook’s access to your info.