Fields and Variables are handy features you can use to get information onto contracts and streamline your workflow. To summarize, Fields can be used for clients to enter information onto a contract themselves and Variables are dynamic fields you can use to automatically populate specific information.
This video explains more about each feature and shows how they can be used when creating and completing a contract. You can use the chapters below to skip to a specific part of the video.
1:01: Custom Variables
2:50: Client Experience
There is also a more general tutorial for creating and sending a contract which you can find here.