A bar is a continuous dark area, a space is a continuous light area.
Most bar codes have a fixed number of possible bar widths.
That is, each and every bar must be one of a fixed number of sizes. Each bar's width must also be a multiple of the smallest bar width. But not, 15mil, or 25mil since these sizes are not multiples of the narrowest bar size of 10 mil. The width of the smallest bar is defined as a bar code's 'X' dimension. Each X' dimension is sometimes referred to as a module.