[3D Secure][0] is an additional authentication layer that some
credit/debit cards use. Typically, users open a browser and do some
authentication check before the transaction can proceed.
This adds 3DS support for credit/debit card donations.
If you want to try this yourself, I recommend using one of [Stripe's
test 3DS cards][1] in a staging build.
See also: [the equivalent commit in Signal Android][0].
[0]: https://stripe.com/docs/payments/3d-secure
[1]: https://stripe.com/docs/payments/3d-secure?platform=ios#three-ds-cards
[2]: d1df069669
