Jquery UI Tooltip, API design is confusing

Jquery UI Tooltip, API design is confusing

Can someone please explain the difference between the two:

Usage 1: ('.selector').tooltip( { items: 'item_that_needs_tooltip', content: "<p>My tooltip content</p>" })

Usage 2: ('item_that_needs_tooltip').tooltip( {content: "<p>My custom content</p>"})

What is this mysterious ".selector" that appears in virtually any example in the API documentation?? It's not explained at all and examples simply set it as 'document'. Documentation says that when I set "content" attribute, I likely need to change "items" attribute as well. Why? It says that content and items should be synced. What does that even mean, what's wrong with Usage 2: setting custom content without setting items attribute at all?

I have not yet started using this widget, I only read the documentation and examples and it's totally confusing.