What does this symbol mean in IntelliJ? (check mark between file icon and file name)

Solution 1:

It is a bookmark. Use menu item Navigate > Bookmarks > Show Bookmarks to show all your bookmarks.

Solution 2:

There are two ways to remove it:

  1. You can click on the folder and press F11.
  2. You can click on the folder and then use the menu item: Navigate > Bookmarks > Toggle Bookmark