Hashing is a process that converts a key into a hash value using a hash function and mathematical algorithm. A good hash function uses a one-way hashing algorithm so the hash cannot be converted back to the original key. Hash tables store data in an array format using the hash value as an index, allowing very fast access if the index is known. Hashing provides constant-time search, insert, and delete operations on average and is widely used for applications like message digests, passwords, and data structures.