Basically it's a way for you to pick up a call before actually picking it up. The way it works is when you receive a call you have the standard pick up and hang up options bit there's an extra button called screen call.

Once you click it google's TTS will say that the call is being screened and that the actual user will see it (it kind of sounds like a voicemail message and a few people confuse it for that but it isn't). Then whatever they same gets turned to text and displayed on screen.

You have a few default responses like Tell me more, I'll call you back, Don't ever call here again (I forgot the specific formatting of this one), and a few others. At any time during the process you can choose to pick up and use it like a normal call or hang up

EDIT: Spelling (dknt to don't)