SHA384 is a cryptographic hash function in the SHA2 family. It takes an arbitrary amount of data and maps it to a fixed number of bits, in this case 384. While it hasn’t been proven, it’s believed to uninvertable, that is, it’s impossible to get back to the original data from its hash.
The SHA2 Family contains two interesting functions, SHA-256 and SHA-512, which are 32 bit and 64 bit version of essentially the same function, while SHA-384 is a truncated version of SHA512 computed with a different initial value.
Since SHA384 is a truncated version of SHA512, why would you prefer it over
SHA512, after all more bits is better right? Well, one you’ve hit 256
bits, you’re already dealing with a phenomenally large output space, so
it’s mostly a matter of how many output bits you need.
See this stackoverflow post for more information. Additionally SHA384 isn’t vulnerable to length extension attacks while SHA256 and SHA512 are.