How to redeem
On iPhone: App Store → profile picture → "Redeem Gift Card or Code" → enter the code from the bot. On a computer the same is done in the Music or App Store app.
Account region
A code only works in the region it was issued for. Your Apple ID region is in Settings → your name → Media and Purchases. Pick a card that matches, otherwise the system will refuse it.
Why you need it
A Russian card cannot pay for iCloud, Apple Music or in-app subscriptions. An Apple ID balance solves that: it is charged automatically on the next payment.