The datasheet for the microcontroller should answer all those questions . . .
but here I go:
VCC (power), AVCC (analog power), AREF (analog reference voltage), PB, PC, PD (ports)
Its more complicated than this, but again, the datasheet should answer everything. It will all become obvious to you as you start to build the robot.
The tutorial obviously cant explain everything
, but it does however go over everything you need to know to build a working robot.