People who are identified as clinically extremely vulnerable may be at high risk of serious illness if they catch coronavirus (COVID-19). They have been advised to take additional action to prevent themselves from coming into contact with COVID-19 when transmission of coronavirus in the community is high.

https://www.gov.uk/government/publications/guidance-on-shielding-and-protecting-extremely-vulnerable-persons-from-covid-19/guidance-on-shielding-and-protecting-extremely-vulnerable-persons-from-covid-19