While Vault provides businesses tons of functionality out of the box, what feature allows you...

Plugin backends are the components in Vault that can be implemented separately from Vault's built-in backends. These backends can be either authentication or secrets engines. All Vault auth and secret backends are considered plugins. This simple concept allows both built-in and external plugins to be treated like Legos. Any plugin can exist at multiple different locations. Different versions of a plugin may be at each one, with each version differing from Vault's version.

Reference links:-

https://www.vaultproject.io/docs/plugin

https://www.vaultproject.io/docs/internals/plugins