The Square Aspect joins planets that are 90 degrees apart (give or take a few degrees of orb).
This aspect typically indicates stress and conflict, but can also be motivating and dynamic. In a natal chart, it indicates a conflict of interest between the two planets that shows as difficulty in the person's life, which often becomes a lifelong quest to resolve the struggle and/or channel into constructive motivation. Most people become better at handling the square as they mature.
Planets that are square to each other are usually three signs apart, and they share the same modality (Cardinal, Fixed or Mutable).