When software with SSL enabled sends a message, it sends it encrypted - that is, in a secret code. The software that receives the message, provided it too has SSL enabled and knows the secret code used, decrypts it - turns it back into the original uncoded message.
This gives you the first of the benefits of SSL mentioned above: confidentiality. If some unintended recipient - known as an eavesdropper, or attacker - picks up the message, they can't decipher it, because they don't know the code.
To encrypt a message, you need a key and an algorithm. A key is a number, and an algorithm is a method of combining this number with your message to create the encrypted message. To take a simple example, imagine you encrypt a message by replacing each letter by the letter two places farther along the alphabet, so that, for example, IBM becomes KDO. Here, "replace each letter by the letter n places farther along the alphabet" is your algorithm, and 2 is your key.
If you were to use a different key, you'd get a completely different code, although you're still using the same algorithm. So if for example n = 1, IBM becomes JCN.