In common usage, destiny means some prearranged life plan, possibly watched over by some higher forces which will make it so the destiny gets fulfilled.
In Fourth Way discourse, destiny is only a remote possibility. Man lives under the Law of Accident: things happen and what happens does not have any transcendental purpose, except maybe the purpose of keeping man where he is and 'feeding the Moon'.
To have destiny, one must be. Destiny is not a given, it is a remote possibility. Some rare persons come to the world with a destiny. These are the great teachers and luminaries of history, Jesus, Buddha and others. Even they must seek and struggle on Earth to connect with their destiny.
We can look at how destiny and free will are not opposites but rather mutually necessary through the Cassiopaean material. The channeled source says "We are you in the future". This implies a sort of destiny in the sense of a path existing from the present to the future. Still, this path is not the only possible path. Following this path must be an act of free will. In an open universe where unconditioned free will is the founding principle of all, there cannot be deterministic destiny. The closest too deterministic destiny there is is the fate of inanimate matter. The more conscious something is, the less determined it is. The regular human is still heavily determined but holds the potential for reaching towards a destiny.
The more one reaches towards the destiny by practical action, the higher the probability of this particular future becomes, one sort of simultaneously builds one's future and is drawn towards it. A similar idea is expressed by Rudolf Steiner when he says that spirit reaches towards the past, whereas matter flows towards the future and experience is their intersection.
Action in favor of one's destiny, we could even say in favor of creating one's destiny is sometimes called DO-ing in FOTCM discourse.