Decap cell
Beginner
A tiny part that stores a little electricity right next to busy parts, like a water tower for the neighborhood.
Novice
A cell with no logic function that is simply a capacitor, a tiny store of electric charge, connected between supply and ground. When nearby logic suddenly needs current, the decap supplies it for the moment before the rest of the supply can catch up.
Expert
Decap answers the first and fastest dip in supply voltage, and at high frequency only helps logic within a small radius, so it goes near busy logic. It costs area and leakage, and adds to the chip capacitance that resonates with the package’s inductance.