"Mobian aims to integrate the standard Debian distribution with Phone-specific projects and modifications in a distribution that works on certain mobile phones and tablets, such as the Pinephone, the Pinetab and the Librem 5. The idea is to minimize the Mobian specific pieces by “upstreaming” changes to the original projects as much as possible."
https://blog.mobian-project.org/posts/2021/02/18/what_is_mob...
And the basic answer is:
"Mobian aims to integrate the standard Debian distribution with Phone-specific projects and modifications in a distribution that works on certain mobile phones and tablets, such as the Pinephone, the Pinetab and the Librem 5. The idea is to minimize the Mobian specific pieces by “upstreaming” changes to the original projects as much as possible."