feditips, 
The "CW" icon on Mastodon stands for "Content Warning". The contents of the post is hidden, and you have to click on the warning to see the contents.
It was originally introduced so people could hide potentially upsetting topics, but it can be used for any purpose.
Many people hide political posts with a "Pol" content warning, while others use CWs to hide spoilers on posts about films.
Some use CWs to tell a joke in the warning, with the punchline revealed in the contents.