using Capacitors to block variations in source voltage?
The power distribution network (PDN) in a PCB is not ideal. It has non zero resistance and inductance.

Let's say an IC needs a sudden increase on supply current. The resistance and inductance of the PDN could cause that such a sudden increase in current consumption generates a temporary voltage drop (a local drop, close to the IC).

A local capacitor, very close to the IC, has the capability to provide the extra needed current until the PDN can keep up to the pace of change and provide that current by itself, recharging in the process the capacitor.