From your Inbox, you can see all the messages you have received. You can also reply directly to these messages from your Inbox.
When in your Inbox, click on a message to expand it.
By clicking on the Reply button, you'll be taken to the Rapid SMS screen where you can reply directly to that customer. If you have a name saved for that customer, you'll see their name instead of their mobile number.
Type in your message, or use a saved template, then hit "Send SMS".