Cart modifiers are simple plugins that allow you to define rules according to which carts should be modified (and in what order).
Generally, this is how you implement a “bulk rebate” module, for instance.
Warning
When using cart.extra_price_fields.append('your label', price) you might want to use from django.utils.translation import gettext as _ for your label in multilingual projects. Please make sure that you use gettext indeed and not ugettext_lazy!