Seal / Unseal
When a Vault server is started, it starts in a sealed state. In this state, Vault is configured to know where and how to access the physical storage, but doesn't know how to decrypt any of it.
Unsealing is the process of obtaining the plaintext master key necessary to read the decryption key to decrypt the data, allowing access to the Vault. Review this article to learn more about the Seal and Unseal processes: Vault Security