Translations:Mastodon features explained/43/en

The last symbol in the row - the cw - stands for content warning. When you click on it a smaller text field will appear above the larger text field.
When you post a toot with a CW people will at first only see what you have written in the smaller text field and have to click to see the larger text.
When you add a picture or video to a toot with a cw then the media will automatically be marked sensitive. You can however click on the checkbox to change that.
Besides warning people about possibly triggers CWs are sometimes also used for jokes - the punchline being hidden.