"tar.gz" is a type of archive file. Similar to ".zip" in Windows.
I'm not a KDE user, but I bet you can probably right-click on the icon and choose "Extract" or "Expand" to take a look and see what's inside the trash.tar.gz archive.
Probably all that happened (this is just a guess) is that you accidentally right-clicked on your Trash folder and then accidentally selected "Compress" or "Archive" from the drop-down menu.
When you say "Worse I can't get rid of it," what steps specifically have you tried? Did you try right-clicking trash.tar.gz and selecting Move to Trash or Delete? Did you try dragging trash.tar.gz to the Trash? Did you try any terminal-based commands?
Do you share your computer with other people, or is it just you? Have you recently run any commands with "sudo" or "as root user"? If you don't remember, you can check with the following Terminal command:
Code:
cat ~/.bash_history | grep sudo