When someone is asymptomatic, this means that they have the virus in their body, but they do not show any symptoms of it. This is dangerous within a society because people who are asymptomatic are not taking the self-isolation recommendations and maybe spreading the virus without realising that they are doing so.
Initial information about asymptomatic individuals from China where the virus originated suggests that in the early days of the virus, up to 85% of total infections included people who had no symptoms or symptoms so mild that they were not associated with the virus. However, any data about the asymptomatic transmission of the virus should be treated with caution because until reliable antibody tests are developed and administered to a much wider population the real amount of asymptomatic people is not known and may not be known for many years.