Liquid particles from the airways can be divided into two categories based on their size: droplets and aerosols, although the distinction is a fluid one. While droplets fall to the ground after a short distance (of about 1.5 metres), aerosols can remain airborne for extended periods of time and can spread or accumulate indoors, especially in confined and poorly ventilated indoor spaces. They are propagated when people breathe and talk, and particularly when they’re laughing, shouting or singing, or in situations demanding increased breathing (such as when playing sports).
The coronavirus generally spreads via droplets and aerosols during close and protracted contact with an infected person. Close contact makes transmission easier because there can be higher concentrations of the virus. With greater distance to an infected person, the virus will be less concentrated, reducing the risk of transmission. This can also be achieved by ensuring that a room or any other indoor space is well ventilated, as this will also dilute the concentration of any virus present.
You can find out how to protect yourself and others, and how important it is to regularly ventilate a room, on the page Protect yourself and others.