Webforms are built using Fields, which can be filled in and submitted by a user. Once a Field is submitted (for example: Name, Email, Date of Birth), the data entered is recorded in the Contact List beside that user's details.
Choosing a 'Field' when creating a webform will allow you to select from a list of the existing fields in the Contact List. However choosing an 'Input' will allow you to create a temporary/disconnected field, detached from any Contact List field. And these 'Inputs' can then be used in conjunction with Calculations.
An 'Input' will appear as shown below, in this example labelled as 'Discount', which is crucial for using in your Calculations. You might also notice the lack of a 'Field' name - usually to the right of the 'Label' - as this Input is not tied to any Field found in the Contact List.
The 'Label' and 'Property Name' here are used to calculate a Discount, and the Input has been Disabled to prevent any editing of the value displayed.