You can easily flag important messages using "Stars" in gmail. You can assign stars to special conversations or messages, or use them as a visual reminder that you need to follow-up on a message or conversation later.
To star a message from your inbox, click the star icon located to the left of the sender's name on a message:
You can also use the keyboard shortcut "s" key to quickly add a star.
To star a message from the message view by clicking the star icon next to the sender's name on a message:
Please note: Stars apply to a single message within a conversation. When you star one message in a conversation, that message will automatically be open when you open the conversation next.
You can search for particular stars using "has:" with the star's name (you can find these names on the General tab of the Settings; just hover your mouse over each star icon). For example, you can search for "has:blue-star", "has:red-bang", or "has:orange-guillemet".
Reference Star important messages