A Stack is a "stack" of blocks that can be included on multiple pages. If you have content that you want to appear on several pages, use a Stack!
For example, perhaps your site footer contains a block of contact information that appears on every page of the site. That contact information is stored in a stack.
A "Standard" stack is created as needed by you, the site editor. A "Template" stack is automatically created by the system from your site design.