Hello Gillian,
You need a iPhone that supports it - I think it was introduced in the iPhone 4. FaceID on other devices provides almost equivalent functionality.
TouchID allows you to use your thumb to confirm your identity rather than a password - it’s much quicker and allows you to use Apple Pay both online and in a number of apps to authenticate yourself. You can use it to pay with things using any contactless shop terminal instead of using your debit card. You can use it on the London underground too should you dare venture that far!
Best wishes,
Stephen
You meet your destiny on the road you take to avoid it. ~ Carl Jung
Morning Smuggers