Starring emails allows you to mark a conversation or message, or use them as reminders for following up at a later time. It will still keep the inbox label or other labels that you have assigned to the conversation until you remove it from the starred area.
To star a message, click the star icon located to the left of the sender's name on a message.
You can also choose to use several types of stars to indicate different types of messages.
To access different star options, follow these steps:
Once you select which stars to use and save them, you will then click on the star to change the colors to set it to the star you want to use:
You are even able to search for particular stars in the search button. If you put has: in the search bar you would be For example, you can search for has:blue-star.