Well, if COVID were bacteria (which it's not), this would be a demonstration of plating. When you have a single bacterial cell and you place it in a nutrient rich medium (like an agar plate) it will replicate. Come back after a few hours and each individual bacterium has turned into a whole colony of many, many millions of duplicated bacteria. So each of those individual dots represents one bacterial cell that was present in the beginning, theoretically. So you can see the ones with more colonies meant more bacteria was being spread.