In case the jargon isn't transparent, in automotive a 'skateboard' is roughly a flat frame with batteries, motor, motor controller, and suspension onto which you can bolt the rest. A vehicle platform, really.
OP is referring to the battery, power electronics and drivetrain. If you picture lifting the body off - sorta looks like a skateboard.
(although cars like Tesla are highly integrated - there's probably still good money to be made in selling the "dev kit")