Certain vehicles either have to meet emissions standards or pay a daily charge when entering these zones. There are penalties if you fail to do this.
You can find more information relating to Low Emissions and Clean Air Zones here, including areas which currently have legislation in place, and those that will be changed soon.
England: https://www.gov.uk/guidance/driving-in-a-clean-air-zone
Wales: https://gov.wales/sites/default/files/consultations/2018-04/180424-clean-air-zone-framework-en.pdf
Scotland: https://www.lowemissionzones.scot/