When a publication is "Offline - only visible to you" it is only viewable when logged in to your publitas account.
Unless someone is logged into your publitas account, they will not be able to access your publication.
When a publication is "Online - public on the web" your publication is accessible by anyone with a link to it.
People can access your publication via the link you find within "Share".
Embed publications on your website from the "Share" option
If you have not disabled SEO, your online publications will appear in search engine results.