The Send SMS block sends an automated sms campaign.
You must first create the SMS campaign from the campaign builder. Then, once you've selected the Send SMS block, you'll see a dropdown of SMS campaigns to choose from.
SMS Campaigns Must be in Ready State to Send
An SMS campaign must be Saved as Ready in order to trigger it through a journey. If it is in Draft state, the SMS message will not send and the next block in the journey will be processed.