Smooth a signal: a schematic puzzle
One resistor and one capacitor let slow changes through and swallow fast noise.
The resistor feeds the capacitor, and the capacitor needs time to change its voltage. Slow signals give it that time, so OUT follows IN. Fast wiggles are over before the capacitor can react, so they barely show at OUT. The hand-off happens around f = 1 ÷ (2π × 10 kΩ × 10 nF) ≈ 1.6 kHz. Picture a bucket under a tap: a steady flow fills it, a quick splash hardly changes the level.
10 kΩ resistor: Limits how fast the capacitor can fill or empty. With the capacitor it sets the time constant, 10 kΩ × 10 nF = 100 µs, and therefore the corner frequency.
10 nF capacitor: The bucket. It holds the output voltage steady and shunts fast changes to ground before they reach OUT.