Send SMS does just that, it sends out SMS text messages from Switch. Now with a selection of SMS providers. It is triggered by a job that passes the Send SMS app in the flow. You have to set up an account with one of the supported SMS providers to use this app. The service is world wide.
Send SMS can be used to notify customers about statuses on their job or for other purposes to make contact with customers. One useful application is to connect Send SMS to a flows Problem Folder to send messages to Switch Administrators that something needs attention on the server.