Someone brought in a mushroom growing in their lawn. I believe the white (sort of) round mushrooms are called puff ball mushroom. They do get very large. All pictured are fungi of course and are prevalent this year due to the frequent rains and high humidity. The source of the mushrooms is spores that can be blown in from other areas. They thrive on decaying material which includes grass thatch, dog droppings and wood from decaying roots or other sources. They prefer a shady area.
Mow the lawn regularly because short grass allows the lawn to dry out more quickly but make sure all the mushrooms and pieces of wood and debris are removed before mowing. When the soil dries out, this should not be a problem. If thatch is a problem, you might want to dethatch.
The mushroom is not poisonous; however, it is uneatable because it is very tough and tasteless.