Effect of aria-hidden in Font Awesome

In short, it makes the icon not visible to screen readers, to enhance accessibility.

From the documentation:

If you're using an icon to add some extra decoration or branding, it does not need to be announced to users as they are navigating your site or app aurally. Additionally, if you're using an icon to visually re-emphasize or add styling to content already present in your HTML, it does not need to be repeated to an assistive technology-using user. You can make sure this is not read by adding the aria-hidden="true" to your Font Awesome markup.