Formal balance means that the weighted elements in a picture or visual display are symmetrical. If a person were to draw an imaginary vertical or horizontal axis through the design, it would appear to have equal weighting of visual elements on both sides. Balance is one of five design principles.
Formal balance is different than informal balance. Informal balance means the objects in the design aren't symmetrical, but the design as a whole still appears balanced. The purpose of achieving formal balance in a design is to give visual imagery an overall appearance of order, as though everything looks in place.