A custom field is a placeholder for data, you cannot fit into Shopify's existing fields for pages, products, collections, etc. available in your Shopify admin detail pages.
Typical custom fields are extra product description fields such as origin details, materials, ingredients and care instructions as well as fields for holding specific information in your area of business, such as product retail prices, specifications, dimensions, weight etc. etc.
You can also create extra fields to control your design layouts such as creating linked products, custom product filters, hiding of specific products or articles from searches, or call-to-action text on buttons.