Plaque assay is a common technique used by virologist to determine the concentration of the virus in a given volume. To understand how the assay works, it is easier to understand the steps behind this technique first.
The first step of plaque assay is to seed cells into a mono layer in tissue culture dish. After seeding, virus is diluted into various concentration. The dilution is usually set up in a way that you would decrease the concentration by ten fold at each dilution. After diluting the virus, you infect the cells that you seeded by simply incubating with the diluted virus. Usually after an hour, the virus is removed and the cells are covered with a mixture of nutrients and agar. This mixture is liquid at the time, but as temperature drops, this mixture solidifies.
Then you wait a day or two for the virus to spread. As the virus spreads, infected cells will be dead and eventually leaving an empty hole (ie. Plaque) as the virus moves outward. At that point, the cells are fixed and stained. And you will be able to count the plaques to determine there plaques formation unit per milliliter.