What is 256-bit AES encryption at rest and in-transit?

Your data is always either at rest or in-transit. When it is at rest it is not actively moving from server to computer network, between computer networks, or so on, and when data is in-transit, it is. Encryption at rest and encryption in-transit means that your data is fully encrypted in both cases.

With AES encryption, both the sender and the receiver of the data must have the same key in order to decrypt and read data. 256-bit AES encryption is the mathematical equivalent of 2²⁵⁶ key possibilities. So both at rest and in-transit, your data is secure.