τ = R · C. See the time constant, settling times, cutoff frequency, and visualized curves — all live.
The time constant τ = R · C tells you how fast a capacitor charges through a resistor. After one τ, the capacitor reaches 63.2% of the supply voltage. After 5τ, it's ~99.3% — engineers call this "fully charged."
The same constant determines the cutoff frequency of a single-pole RC filter: fc = 1 / (2π·R·C). Below fc, signals pass; above fc, they're attenuated.