Mobile Passport comes from the State Department and is used when entering the US. You load your family's passport information into the app and have it accepted. When approaching the US, you fill out the customs declaration on the app. When the plane lands and you can use the cell phone, you hit the transmit key. When the app accepts your data, a Q/R code appears on the app.
When going through passport control you scan the app and hand your passport to the customs officer. Your passport is returned and you're done with passport control. You use the same Q/R code at customs.
It's been a while since I've used this. There are special lines for Mobile Passport users; I've been directed through the diplomatic line at Newark. I do have Global Entry but find Mobile Passport much easier. Better yet, it's free.