Basically ssl is used to encrypt files/data. In combination with TLS you can use it for encrypted connections between hosts (e.g. secure login via https).
To encrypt a file you could use openssl (test.txt is a text file with the content "test"):
openssl enc -aes-256-cbc -in test.txt -out test.enc -pass pass:test
After this a file is created named test.enc which is not readable anymore.
The file can be decrypted using the provided password:
openssl enc -d -aes-256-cbc -in test.enc
enter aes-256-cbc decryption password:
For more information about TLS (Transport Layer Security) check http://en.wikipedia.org/wiki/Transport_Layer_Security