We suggest you read the TIME CONSTANTS before tackling this one.
The integrator consists of a capacitor and resistor connected as shown.
A PULSE TRAIN is applied to the input.
When an input pulse rises rapidly to maximum the capacitor charges exponentially through the resistor as shown in the lower waveform.
When the input pulse falls suddenly to zero the capacitor discharges exponentially to zero.
The process is repeated for each pulse giving the waveform shown.