Intensive care units (ICUs) are specialist wards within a hospital that look after people who are very ill.
They're equipped with a lot of highly technical monitoring equipment and staff who've been specially trained.
Coronavirus patients have problems with their lungs and so are usually admitted to ICUs so they can get help breathing.
As Prime Minister Boris Johnson continues to be treated for coronavirus in an ICU, BBC health correspondent Laura Foster explains how they work.
Video by Tobias Chapple, Terry Saunders and Laura Foster.