Why does super+tab mean?

[luofeiyu]

I am in debian9.
For my emmetoneline key binds setting:

[
{
“keys”: [“super+tab”], “command”: “emmet_css_from_one_line”
}
]

Why does super+tab mean?
Which two keys will i press at the same time?
I can’t find super key in my keyboard.

[acheronfail]

super refers to either the command (⌘) key on an Apple computer, or the Windows (or meta) key on a standard keyboard.