Need help for Photoshop step backward hotkeys!!!!

I have been trying to find out which hotkeys to use in order to properly step back so I can use certain tools better..

The problem is I can't seem to find any answer as to what key is represented by the symbol in front of the 'command + Z' that I have highlighted here on this screen shot!



I made an account just to ask this simple yet comfusing/frustrating question..

Any help is appreciated, thanks!

Option

It's delete, instead of holding down three keys it's just delete.

The symbol you highlighted is the option key. Ignore the message above.