Publish to Web allows you to share a report with people outside of Power BI. This provides an embed code that can be used to put into a blog post or website. This is just an iframe.
Using this option provides no authentication to the report. Whoever has the URL can see the report. Be very careful about what data you share using this option.
This option also has some limitations. If you don’t see the option to publish to web, it is probably due to the fact that you are hitting one of those limitations. Be sure to review the official documentation for a list of limitations.
Official Documentation – Publish to Web