A hydroxyl radical is a reactive molecule formed by one oxygen and one hydrogen. They react (oxidise) extremely easily with almost all of the chemicals found in our troposphere. This removes these chemicals from the air - the oxidation of these chemicals results in water and organic radicals, which an extremely important part of the ozone formation cycle. Essentially, hydroxyl radicals scrub VOCs and other harmful substances from our air, and leave important organic compounds in their place.
Hydroxyl radicals are formed naturally in our atmosphere through the sun’s radiation and water vapour, and react almost immediately after they’re formed. This is often why hydroxyl radicals aren’t found indoors - they usually react before they can get there.