It is a 3D-Secure payment feature within the Teya Rewards (formerly Yoyo) app which is an additional security protocol used to authenticate users.
This authentication step uses a number of methods to ensure you are who you say you are, while greatly reducing the chances of fraud.
At the point of linking your card or making a payment, you may be redirected by your bank to complete an authentication challenge. This is usually in the form of an OTP (One Time Pin) or something similar and it is often required that you complete the challenge within your banking app.
If completed successfully, you would be considered authenticated and may return to the app to finalise the card linking process or payment flow.
NB: When linking a card, you may notice a nominal charge come off your account. This is simply used to ensure the card is still active and this amount will be refunded immediately.