As it pertains to this article, we will focus on the left hand side as you can enable a form field to be required when you setup the form. Lets say you want to make the Your Mobile Number field required.
To do this, you'll need to identify that label's path. In this case it's the label for content_mobile_number.
You can now target this by adding:
This will now display the * to distinguish it as required.
Please note that this is only a visual experience, if you wanted to ensure they entered their mobile number, additional programming will be required.