You can think of it as a precursor to LDAP.

It was a way to provide centralized access to User/Groups/Password and hostnames information over the network. YP was a staple of most UNIX deployments for some time, before being replaced by LDAP and DNS.

You'll find some historical context here [1].

[1] https://en.wikipedia.org/wiki/Network_Information_Service

Sibling comment links to the relevant Wikipedia page, but most people who've dealt with YP probably know it as NIS (turns out using a trademarked term wasn't ideal). https://lwn.net/Articles/874174/ is a discussion of the current state of the world - Fedora is considering removing support for it in a future release, but it's otherwise still hanging around in Linux distributions.