Another very common configuration is combining a resistor and a capacitor in a series circuit. As you saw in the capacitance section, a capacitor stores charges for use later on. The capacitor is not charged instantly, instead it takes some time for enough particles to build up inside it to consider it charged.
Also, as it gets charged, the electrons inside the capacitor will push back the other electrons trying to get inside, effectively creating a voltage. This voltage prevents electrons from flowing as fast as they would otherwise move, thus reducing the charging rate as the internal voltage of the capacitor increases to match the voltage from the source.
But how is it that a unit of resistance multiplied by a unit of capacitance gives a unit of time?, let's look at how the units are defined in terms of charge and voltage:
R = V / I, and I = Q / t, so R = [V t] / Q
C = Q / V
R C = t [V / Q] [Q / V], so R C = t
Where Q is charge, t is the unit of time and V is voltage.
This result specifies the time it takes for the capacitor to go from 0% to 63% of the source voltage, it is also the time it takes to go from 100% to 37% of initial voltage.
This time is called the RC time constant because the time doesn't vary with different input voltages, and varies very little with temperature due to variations in the resistance of the resistor. This makes this combination of components very useful in timing, delay circuits, oscillators and many other applications.
