Gmail blog introduced new feature Inbox preview in Gmail Labs which shows preview of Gmail inbox while loading Gmail. This feature which helps users to check whether any new email is recieved, while Gmail is loading.
To enable Inbox preview move to Gmail labs settings in your Gmail account find out inbox preview option and enable as shown below picture.

After enabling you can view inbox preview after signing inside your Gmail account as shown below picture.

Inbox preview displays an inbox messages in a table of content.