what is an apple ID credit? how do i redeem it?

Where are you seeing this credit, and what do you mean by 'how do I redeem it' ?

If you've received a gift card and want to redeem it onto your account : Redeem App Store & iTunes Gift Cards, Apple Music Gift Cards, and content codes - Apple Support

If by 'redeem it' you mean spend it then find something in the store and buy it - an account's balance should automatically be used for your purchases, and credits (e.g. song credits) should automatically be used for that type of item.