Favicon or Site Icon is the tiny image that appear next to your website title in the browser. It helps your users identify your website and more frequent visitors of your site will build an instant recognition for that tiny image. This increases your brand recognition and helps you build trust among your audiences.

The fastest and easiest method is to use the default WordPress setting.
First access the WordPress Customizer by going to Appearance > Customize:

For most themes you should navigate to the menu option which says Site Identity:

In some themes, this option will say something different to Site Identity.
No matter what, the option should look similar to one of those above. Next, you’ll get the option to upload your site icon/favicon:

Once uploaded, click Save & Publish – then you’re all done.