In a few words, cryptography is the science of hiding information. More specifically, modern cryptography makes use of mathematical theories and computation to encrypt and decrypt data or to guarantee the integrity and authenticity of the information.
In a basic process of text encryption, a plaintext (data that can be clearly understood) undergoes an encryption process that turns it into ciphertext (which is unreadable). By doing this, one can guarantee that the information sent can only be read by a person in possession of a specific decryption key.
By using specific cryptographic techniques, one is able to send sensitive data even over unsecured networks. The level of encryption will depend on the degree of protection that the data requires. For instance, the type of security used on regular personal files (like contacts) is not the same as the one used on cryptocurrency networks.
Learning how cryptography works is critical to understanding its importance within cryptocurrency systems. Most blockchain systems, such as the one of Bitcoin, make use of a particular set of cryptographic techniques that allow them to function as a decentralized and public ledger, through which digital transactions can take place in a very secure way.