To understand about secret-key cryptography, let's consider an encryption algorithm in which each letter is replaced by other letter, for eg., all As are replaced by Qs, all Bs are replaced by Ws, all Cs are replaced by Es and so on like this.
Plaintext = ABCDEFGHIJKLMNOPQRSTUVWXYZ
Ciphertext = QWERTYUIOPASDFGHJKLZXCVBNM
Here, the encryption key is QWERTYUIOPASDFGHJKLZXCVBNM
Now, for the above key, the plaintext CODESCRACKER would be transformed (from the above encryption key) into the ciphertext EGRTLEKQEATK.
The decryption key tells that how to get back from ciphertext to plaintext or original text or normal text or unencrypted text.
Therefore, in the example above, KXVMCNOPHQRSZYIJADLEGWBUFT is the decryption key because an A is ciphertext is a K in plaintext, a B is ciphertext is an X in plaintext, etc.
Many cryptographic systems have property that given the encryption key, it is easy to find the decryption key and vice-versa. Such cryptographic systems are called as secret-key cryptography.
Secret-key cryptography can also be called as symmetric-key cryptography.