I believe it is the idea of writing small tools focused on doing one thing well with reusability in mind as opposed to writing larger complicated tools that do multiple things.
A cargo cult philosophy never adopted by commercial UNIX clones and adored by UNIX FOSS, where the man page of each GNU tool describing the available set of command line arguments looks like a kitchen sink.
I believe it is the idea of writing small tools focused on doing one thing well with reusability in mind as opposed to writing larger complicated tools that do multiple things.
https://en.wikipedia.org/wiki/Unix_philosophy