These are health insurance cards covering certain types of healthcare in certain countries.

For UK residents, details of these can be found on https://www.nhs.uk/using-the-nhs/healthcare-abroad/apply-for-a-free-uk-global-health-insurance-card-ghic/

Each dialysis Centre registered with DialysisAway will confirm any applicable treatment costs for EHIC or GHIC card holders.

You should always take out adequate travel insurance for existing medical conditions when you travel, regardless of whether or not you hold a EHIC or GHIC card.