Groundwork Section III
The number one thing Kant talks about in this section is freedom. He says freedom in regard to will is not something that acts in accordance with the laws of nature, but is not necessarily lawless because of this. This paragraph is kind of hard to understand for me (4:446), but I think Kant is writing that freedom is simply a cause to the effect of whatever laws or maxims are occurring. Freedom is an agent in moving things along, morally speaking.
In this section, Kant writes that freedom is used to support the wills of rational beings, but this support is not yet proved according to him. Kant also claims that, if humans (rational beings) are free, and if reason can motivate moral action, then the categorical imperative is binding.
From this section of Groundwork, Kant emphasizes that is is plausible to think that rational beings are free and that reason plays a role in moral action, despite the fact that causalities appear in the empirical world. Again, Kant thinks freedom in regard to free will has not been proven and thinks it is nearly impossible to do so, but still could be quite possible.
When beings think and act for themselves and make decisions on their own, one can presuppose that they are a free being. Similarly, a part of being a rational being is the use of reason in moral situations. Beings are motivated by reason, but again, Kant does not think this can be proven.
Even though these things (freedom and reason as they apply to moral persuasion) cannot be proven according to Kant, he thinks it is still advisable that beings act as if they could be proven. Without this, there would be no basis for morality.