arrow/exclamation symbol on top bar
This is the Update Notifier signifying that there are updates ready for your system. If you click on it, you'll get some options about these updates, similar to the ones in this screenshot (taken from this question):
Searching for "software-update" icons in /usr/share/icons/, I found (based on the icons' names) that the red explosion icon 
is used to signify urgent updates, while the orange explosion icon 
signifies available updates.