The message you created and want them to understand is plaintext.
You don't want anyone else to know what you have to tell. That makes it a secret.
Someone else is trying to learn your secrets (or you are trying to learn someone else's secrets). That's spying.
The art of communicating in secret is cryptography.
Cryptography converts plaintext into something impossible to understand called ciphertext.
The device, scheme, or trick used to change plaintext messages into ciphertext and back again is called the key.
When your secret communications can not be understood by people whom you don't want to understand them, they are secure.
The art of learning someone else's secrets through understanding their communications is cryptanalysis. That makes cryptanalysis a special type of spying. Cryptanalysis as an art is almost as old as cryptography.
This short document will introduce you to the various methods used to communicate in secret, and their weaknesses as discovered by cryptanalysts. It will also point you in the right direction to develop your own tools for secret communication, that will remain secure against even the most determined assault by Nameless Secret Agents.