'Proleptic' means anticipatory - as most commonly used in grammar, it indicates something taken in advance of its rightful place.
Proleptic reasoning allows us to make rational choices about our lives on the basis of reasons which we know aren't exactly the right ones. When we undergo a transformative change, the values that we are changing in order to truly value are not actually knowable to our current selves. In traditional decision theory, therefore, all transformative choices are essentially irrational. Yet while we do not know the exact values of our aspirational future selves, we do not know nothing about them, either. So a proleptic reason is an approximation of our future reason for the sort of person we want to become. It is a consciously provisional reason holding the place for the unknown-but-guessed-at values of their later selves.
I admire this term because it asserts human agency, based on our present best judgment, in the difficult process of beginning anything new. I am reminded of Hannah Arendt's concept of natality (the possibility of politics wrought by the fact that human lives begin), and of her concept of Action as something which can be begun but whose consequences can never be predicted. To my mind, Callard adds a valuable idea to that theory of action: while the sort of people we are going to become cannot be predicted with any accuracy, it is vital that we see the possibility of directing ourselves towards the sort of people we think we want to become.