Transiting Chiron square your natal Ascendant is a period when Chiron, the asteroid that marks your oldest wound, sits 90 degrees away from your Ascendant, the point in your chart that describes how you come across to the world before anyone knows you. A square is two needs that will not fit in one day, and this one pits the wound against the face. What tends to rise is an uncomfortable question about whether the person others see is really you, and whether the way you present yourself has been shaped more by old hurt than you realised.
Because Chiron turns retrograde every year, it often crosses the exact square three times before moving on. The transit lasts several months and can stretch past a year, so this is not a quick sting. It is a slow pressure.