The Pinephone is "an open source smart phone supported by all major Linux phone projects." I'd been keeping my eye on these things for a few years now and finally got the Pinephone in my hands earlier this week to try it out.
The idea with devices like this (as well as the Librem 5 phone) is that it's a smartphone that runs standard mainline GNU/Linux software instead of Android. This means root passwords (or sudo), SSH servers, full admin control of the device, and ability to run all the familiar Linux software that you get on a desktop PC, server, or Raspberry Pi type of device.
Importantly this means no Google Play Services or built-in spyware and the user is in control of their device. More privacy, but not necessarily more security.
See also: I'm maintaining a Linux on Phones page on my site's wiki to collect notes, config tweaks, software I found that works and so on.