This is a tool to manage encrypted local backups of secrets. The core idea is that it aims to be usable without depending on it, meaning that even if the software disappeared from the face of Earth tomorrow, your data would still be recoverable.
It also integrates nicely with NixOS (which is what I use, though it does not require NixOS to be used).
I have summed up a bit of explanation and some answers to reasonable questions in a blog post: https://baldino.dev/blog/secs-man/