Many people understand encryption to mean only “end-to-end” encryption, which ensures that no one other than the respective participants can (co-)read the exchanged content. However, encryption is much more than that, because secure and traceable encryption (cryptography) includes so-called protection goals:
Beyond these protection goals of cryptography, however, depending on the requirements, inconsequentiality to third parties is also a very important point. This is to be achieved by means of “perfect forward secrecy” (external). This ensures that even someone who eavesdrops on and stores the encrypted communication cannot decrypt it if they later learn a key.
… is the encryption technique developed as “Axolotl” (external; PDF file) by Signal. Between two endpoints (hence “end-to-end encryption”), the previous encryption process is taken as the starting point for each new encryption process - and the ratchet is turned a bit further, as it were.
The Signal encryption technique with the double ratchet is the basis for many other implementations such as OMEMO (XMPP), OLM/MEGOLM (Matrix) or others.
A great site with introduction and special knowledge about cryptography: https://kryptografie.de/kryptografie/index.htm (extern)