Can someone tell me what this white card is?

[Catnap26]

I recently had to have an emergency appointment with a dentist and I asked how much it would cost.the receptionist asked if I received any benefits.i said only child tax credits.she asked if I had a white card (???) I said no so she said i would have to pay the full cost.
My question is what is this white card and how do I get one/qualify for one?

Nhs exemption certificate by the sounds of it. It's a white credit card sized card that gets sent to you automatically if youve claimed working/ child tax credit and your income is below £15,700ish, can't remember exact figure.

Low income NHS costs

Thanks so much.ive tried to google it but with no luck.i shall do some researching now

You will get it automatically if your income is under about £15000 and you get tax credits :-D