What does "Guardians Make Their Own Fate" mean?

In Destiny, the vast majority of beings are causal, meaning one event leads into another in a way that can be predicted. This is the basic concept of the Vex simulations. However, some beings enhanced by the Light or the Darkness (i.e. Guardians) are paracausal, meaning they cannot be simulated by the Vex. "Guardians make their own fate" refers to the paracausality of Guardians.

Found this excellent comment on the Destiny subreddit:

"Guardian's make their own fate" Is about Kabr and how he created the Aegis and who guardians are. Kabr's fireteam was wiped from existence except for Pahanin and Praedyth. Kabr forged the Aegis by sacrificing himself. Instead of the vex wiping him from existence he "pierced" the vault knowing that if he died he would at least make it worth something. The Aegis is the only thing that saves us in the Vault.

"Guardians Make Their Own Fate" Is a reminder of Kabr's sacrifice. It is a reminder that we carry the mantle.

"I have destroyed myself to do this. They have taken my Ghost. They are in my blood and brain. But now there is hope. I have made a wound in the Vault. I have pierced it and let in the Light. Bathe in it, and be cleansed. Look to it, and understand: From my own Light and from the thinking flesh of the Vex I made a shield. The shield is your deliverance. It will break the unbreakable. It will change your fate."

"Guardians make their own fate" is a reference to Kabr's creation of the Aegis. Anyone that entered the vault was "fated" to die/be erased from existence. As this process was happening to Kabr, Pahanin, and Praedyth, Kabr sacrifices himself to create the Aegis, giving any guardians that follow a way to defeat the vault and stop that "fate". Kabr was creating a way for guardians to "make their own fate". So when we use the Aegis to go into the past and future, it is creating our own fate to defeat Atheon.