According to the National Institute for Health, the diameter of a Coronavirus is approximately 100 nano microns. In humans, they are typically transported airborne in droplets of approximately 500 nano microns or less.
There are voids between the weft and warp yarns in fabric. This allows for air permeability and the ability to breathe while wearing a cloth mask. The average diameter of the void in common fabrics is 200-300 nano microns.
By comparison, the void diameter of the strings of a tennis racket is approximately .25 of an inch, so a cloth mask has the same proportionate ability to block a coronavirus as a tennis racket would to block a pencil eraser.