Capacitor's act as differentiators. Their current is proportional to the rate of voltage change (dV/dt) across their terminals.
In the circuit on the left, the capacitor's current is positive as the input's voltage rises and negative as the voltage falls. This positive/negative current reflects the *slope* of voltage over time.
In practice, a resistor is added to this circuit to sense the current as depicted by the circuit on the right. In this circuit, the voltage at the resistor would be roughly proportional to the signal's rate of voltage change.
Unfortunately, adding this resistor means the capacitor's current is now proportional to the difference between the signal being measured and the point of measurement (resistor acts as voltage divider). Keeping the resistor/capacitor (RC) values small provides a useful circuit.
