Encryption scrambles a message so that only someone with the right key can unscramble it. The scrambled version is called ciphertext; the original is plaintext.
The simplest kind uses one key for both directions: the same key locks and unlocks. This is symmetric encryption. Think of a padlock where everyone who needs in has a copy of the same key.
It is fast and strong. Its problem is distribution: how do you get the key to the other person safely? If you could send a key safely, you could have just sent the secret safely, and you would not need encryption. This is the chicken-and-egg problem that the next idea solves.