Google ANDROID 2.3.4 - Manual (Page 153)

Gmail 153
AUG-2.3.4-105 Android User’s Guide
Starring a message
You can star an important message to make it easy to locate again. Conversations
with starred messages display a star in your Inbox and other conversation lists. To view
just conversations with starred messages, see “Viewing conversations by label” on
page 155.
Star a message
S While reading a message, touch the star in its header.
OR
S While viewing a list of messages in a conversation list, touch a message’s star.
The star turns gold .
Unstar a message
S Touch its star again.
Unstar all messages in a conversation
S Touch the conversation’s star in a conversation list.