Cryptography or encryption is the method of disguising and revealing, aka encrypting and decrypting, information through complex mathematics. This means that the information can only be viewed by the intended recipients and no one else. The method involves taking unencrypted data, such as a piece of text, and encrypting it using a mathematical algorithm, known as a cypher. This produces a cyphertext, a piece of information that is completely useless and nonsensical until it is decrypted.
In blockchain, cryptography is primarily used for two purposes: securing the identity of the sender of transactions, and ensuring the past records cannot be tampered with.