This hook's headSting represents everything that will be written to <head> tag.
There are many possible SEO uses to this hook, especially for plugins. That said, we recommend users who want to set common SEO elements such as tags <title> and meta descriptions programmatically to do it from within Svelte templates using the <svelte:head></svelte:head> tag. Chances are you won't need this field unless you're a power user and need access to the raw head.