VDD and VSS
Beginner
The two power connections every part of a chip needs, like the plus and minus ends of a battery.
Novice
VDD is the positive supply voltage, below 1 V in many modern chips; VSS is ground, 0 V. Every logic cell connects to both, and the current it uses flows in from VDD and out to VSS.
Expert
Both are networks with resistance. A cell sees its local VDD minus its local VSS, so a sag on the supply side and a rise on the ground side (ground bounce) come out of the same voltage budget, and both nets need analysis.
Explained in Power planning (Design Flow).
See also: Power delivery network (PDN), IR drop.