Stronghold is an open-source software library that was originally built to protect IOTA Seeds, but can be used to protect any digital secret.
It is a secure database for working with cryptography, which ensures that secrets (like private keys) are never revealed - but can be used according to best practices.
It provides its own peer-to-peer communication layer, so that different apps can securely communicate using the state-of-the-art Noise Protocol over libp2p.
This blog published for the beat release of stronghold gives great insights to readers.
A general deeper introduction in the concepts is layed out in the introduction article
Read more about how Stronghold secures applications and how you can use it to build securely with it in the documentation