SHA-512 is a message-digest algorithm. It's used to compute a hash value in cryptography. So what's an hash function then? Simply put, a hash function takes a block of data and returns a fixed-size bit string (hash value). The data used by hash functions is referred to as a "message", while the computed hash value is referred to as the "message digest".
SHA-512, like other hash functions, is used in digital signatures, message authentication codes, to index data in hash tables, for finger-printing, to detect duplicate data, uniquely identify files, and as checksums to detect accidental data corruption.
SHA-512 produces a 512-bit (64 bytes) hash value. It's usually represented as a hexadecimal number of 128 digits.
You can't! SHA-512 is NOT an encryption algorithm! A lot of people are under the impression that SHA-512 encrypts data. It does no such thing. All it does is compute a hash value for a given set of data.