Canned Messages are pre-defined messages in your account. For instance, a canned message might be an answer to a frequently asked question. During a chat, an agent can pick out canned messages and send them to the visitor, which helps the agent provide the visitor quick response.
Note: You can create either a public canned message or a private one.
There you are. The new settings will take effect immediately. You do not need to re-paste the live chat code to make the change work.