It's like the contacts app in your smartphone where you keep the phone numbers of people so you don't have to remember them. The main difference is that a phone book has lots and lots of people so you can look up numbers for people you have never met. We used to get updated versions as printed books every year or so. These days, they still exist as apps or websites.

My brother-in-law is 10 years older than me and he was trying to explain to his kids what dial-up internet was like and what it actually was. We covered the speed and the sound and he mentioned it was internet over a phone line. I had to remind him "they don't know what a 'phone line' is!"