Regular patterns from geometric shapes are thought to indicate an organised and efficient mind. These shapes are also familiar from elementary mathematics, so are easily drawn and an natural extension of simple mark-making, which could be considered contrary to that interpretation. They can also be very symbolic, so should always be interpreted in context.
The circle appears in every culture as an archetypal form representative of the eternal whole. With no ending or beginning, it revolves in an eternal cycle and is linked to the sun-disk and the attendant concepts of the yearly cycle, the moon, and the wheel, so it's often used to represent the sun (especially with rays) or the full moon. In some symbol systems it also represents the universe.
The square represents the formal, mathematical, scientific order of the universe. The square represents earthbound matter, and correspondingly, with its two sides delineating a two-dimensional surface, may symbolize the earth or ground, or a field, especially in eastern pictograms. In Buddhist symbolism the relationship the square within the circle represents the relationship of the human and the divine.
In religious symbolism the triangle represents the trinity. In pagan symbolism the upward-pointing triangle can represent the blade or sword and is masculine in quality, and also the astrological fire signs, while the downward pointing triangle represents the chalice or cup, feminine in quality, and the astrological water signs. The three sides of the triangle make it very stable, particularly with its base immovable on the ground. Its geometrical stability suggests purpose. It's also used in many modern symbol systems including warning signs, and the inverted pink triangle of Gay Pride.