Only you can see your own "Starred" messages. This acts as your own personal bulletin board.
What is "starring" a message
Starring a message is a great way for you to store a message in a separate space that only you can view. You can access it at a later time by clicking on "Starred Messages" under your channel menu. Nobody else can see your starred messages but you.
How to star messages:
Hover cursor over the message you wish to star.
Towards the right side of the message being highlighted, you should see a vertical three dots icon.
Click on the icon to reveal the dropdown menu.
Select "Star" and this will star that message.
To view starred messages:
Open the channel you wish to view the starred messages on.
Towards the top right corner of your chat space, click on the icon with three dots.
Select "Starred Messages" on the dropdown and a list of your starred messages in that channel will pop up.