A deal is where we store everything related to an arrangement to work with a customer. Right now that's Proposals (Quotes/Estimates) and Invoices, but we'll be adding a lot more soon, for example:
Scheduled appointments or job bookings
Custom forms specific to your industry
Communications with the customer
Reminders and tasks.
Ensuring everything is available in a single place will help you keep everything organised and accessible, it'll save you a lot of time make your life simpler.