Transiting Pluto square your natal Ascendant is Pluto, the planet of power, endings and what goes deep, sitting 90 degrees away from the point in your chart that describes how you come across to people. A square is two needs that will not fit in one day: in this case, Pluto's insistence that something change at the root, pressing hard against the Ascendant, which is the face you show before anyone knows you. The transit lasts two to three years, often crossing the exact degree three times as Pluto turns retrograde and moves back and forth across it.
Because Pluto takes about 248 years to go round the zodiac, it makes this square to any one natal point twice in a lifetime, at most: once as it moves away from the conjunction with your Ascendant, and once as it comes back towards it again. Most people feel this transit once. The Ascendant depends on your birth time, so an accurate birth time is what makes this transit meaningful rather than approximate.