SHA-2 (Secure Hash Algorithm 2) is a set of cryptographic hash functions designed by the United States National Security Agency (NSA). Cryptographic hash functions are mathematical operations run on digital data; by comparing the computed “hash” (the output from execution of the algorithm) to a known and expected hash value, a person can determine the data’s integrity. At this time, SHA-1 is acceptable. However, with the continual increase in computing power the security of SHA-1 will become a concern in the future. DigiCert recommends that you accelerate SHA-2 deployment where possible and prepare to migrate fully to SHA2.
For more details check the FAQ for this algorithm.