In a very general way, encryption refers to a method of applying special mathematical processes to your data that scramble the data in a way that can only be understood by a computer that knows how to unscramble the data using the same process and the same formula.
Encrypted flash drives protect your information by requiring a “key” to access any information. Without the key, data stored on the flash drive cannot be meaningfully retrieved.
Encrypted hard drives are a way to protect your data from being accessed without your permission. There are many options including hardware or software encryption with 128-bit to 256-bit encryption levels.