By default, when a new subscription is created in Stripe, they are immediately charged for the first payment of that subscription (as long as there is no free trial). If that charge cannot be processed immediately - e.g. because the card requires 3D Secure verification - then the subscription fails to be created, which prevents the submitter for successfully submitting the form.
Turning this setting on, will create the subscription even when the payment can't immediately be processed, which will allow the submitter to continue regardless.
See this article on 3D Secure Stripe Subscriptions for more information.