Custom Fields for Opportunities enable users to "tailor your CRM to match your exact sales process" by tracking business-specific details beyond default fields.
Custom Fields allow you to "add, manage, and organize additional data points in your sales pipeline" including text, dropdown, date, and monetary field types organized into folders.
Via Settings:
Navigate to Settings menu and select Custom Fields
Via Opportunity:
Inside Opportunities, select an opportunity and click "Add/Manage Fields"
Click the "+Add Field" button at the top right of the Custom Fields dashboard.