Input Variables are submitted by the game and can be used by the Command Sequence. For example, when logging in a user, the game will send the username and password entered into the UI InputFields. These username and password are input variables.
Input Variables must be sent when a Command Sequence is told to run through code. See how to include them here.
Input Variables can be added by selecting the start node in a Command Sequence, going to the 'Input' tab in the properties panel and adding the input variable name to the list. Remember the order of the input variables as they must be added to the run Command Sequence code in the correct order.