Copy past directly from WHO
Based on what we currently know, transmission of COVID-19 is primarily occurring from people when they have symptoms, and can also occur just before they develop symptoms, when they are in close proximity to others for prolonged periods of time. While someone who never develops symptoms can also pass the virus to others, it is still not clear to what extent this occurs and more research is needed in this area.
Limiting contact with people who are infected with COVID-19, frequent, thorough, hand cleansing and wearing a mask when at least 1 metre of physical distance can’t be guaranteed, help to break chains of transmission.
Also from WHO
Yes, infected people can transmit the virus both when they have symptoms and when they don’t have symptoms. This is why it is important that all people who are infected are identified by testing, isolated, and, depending on the severity of their disease, receive medical care. Even people confirmed to have COVID-19 but who do not have symptoms should be isolated to limit their contact with others. These measures break chains of transmission.