Gravatar stands for Globally Recognized Avatar. It's a web service that allows people to create a profile and associate avatar images with their email address.
All WordPress sites have built-in support for Gravatars and will automatically show the user's avatar when they write posts on a multi-author WordPress blog or leave comments.
If a user doesn't have a Gravatar, then WordPress will show a default Gravatar instead. For most WordPress websites, this is a grey mystery person.
You can change this default Gravatar to any other picture of your choosing. For example, you might use an image that represents your company, such as your website logo. This can help reinforce your branding.