When you first pour hot water over ground coffee beans, a substantial amount of carbon dioxide is released. This de-gassing process can disrupt the contact between the grounds and the water, giving you under-extracted coffee.  By saturating the grounds with a small amount of water and waiting, you give time for the gas to escape. This gas bubbling up like foam is what is referred to as the bloom. Then you can pour the rest of the water and get optimal extraction.