a decoupling capacitor
A decoupling capacitor is a little local water tank placed right beside a thirsty machine, so that when the machine suddenly gulps a big draught, the long pipe back to the pump never sags. A digital chip is exactly such a machine: every time its gates switch, it grabs a sharp spike of current. The thin copper traces feeding it have inductance and resistance, so without a local reservoir that spike would make the chip's own supply voltage dip and ring. A decoupling capacitor sits straddling the chip's power and ground pins and supplies that instant gulp locally.
It works because a capacitor resists sudden changes in voltage; it holds a little charge ready to dump in a hurry. When the chip demands a fast pulse of current, the nearby capacitor delivers it within nanoseconds, far faster than the slow, inductive round trip back to the power supply. The classic value is 100 nanofarads (0.1 microfarad) per power pin, placed as close to the pin as physically possible, because the loop inductance between cap and pin is what limits how fast it can help. Often a larger bulk capacitor, say 10 microfarads, handles the slower, bigger demands while the small 100 nF handles the fast edges.
This is the single most important practical habit in all of circuit building: one small ceramic capacitor bridging every chip's supply pins. Skip it and a schematic that is perfect on paper will glitch, reset at random, or radiate interference, because the switching current is forced into a long loop and turns into both supply noise and an antenna. Honest nuance: the capacitor and the trace inductance together form a resonance, so above some frequency a capacitor actually behaves as an inductor and helps less. That is why short connections, and sometimes several values in parallel, matter far more than chasing one big capacitance number.
Put a 100 nF ceramic across the VCC and GND pins of a logic chip, within a couple of millimetres of the pins. Without it, the chip's local 5 V rail can dip 0.5 V on every clock edge and cause random misbehaviour; with it, that dip nearly disappears.
Right at the pin, every chip — the habit that makes paper designs actually work.
A decoupling capacitor's exact value matters far less than its placement. A 100 nF cap soldered millimetres from the pin beats a 10 microfarad cap a centimetre away, because the connecting inductance, not the capacitance, sets the high-frequency performance.