In practice
Mass is how heavy a solid form looks or feels. Dark, large, dense and closed shapes read as heavy; pale, thin and open ones read as light. Designers use mass to anchor compositions and shape how products feel: a thick concrete bench seems immovable, while a thin wire chair seems almost weightless. On a page, a heading in black, heavy type carries more visual mass than the same words in a light weight.
Often confused with Volume
Volume is the three-dimensional space a form occupies; mass is how heavy that form appears, which also depends on darkness, density and material.
Try this
Set one word in the thinnest and heaviest weights of a typeface and balance them on a page by adjusting size.