Solutions allow the user to create predefined line items inside of opportunities as well as using them inside of the document builder.
To create line items select "Account" > "Account Settings" > "Customizations" > "Solutions"
Once the "Solutions" category has been opened select you can choose to "Add New Solution" this will bring up the solution creator where the user can define the Name, SKU, Categories, Description, Product Image, Price, Cost, Margin, and Billing Cycle. Or you can select "Upload CSV" where you can upload a CSV file of all of your solutions.
Once the solution(s) have been created they can now be added to an opportunities and documents (proposals, etc.)
Learn more about creating opportunities here: Create, Edit, and Delete Opportunities
Learn more about creating documents here: Document Builder