Telos is the Greek philosophical word for the final goal of a thing — what it is for, what its full realisation looks like. The standard ancient Greek view, shared by Plato, Aristotle, the Stoics, and (in modified form) the Epicureans, is that human life has a telos, and the philosophical project is to identify it and align life with it.
The Stoic formulation, attributed to Zeno and refined by later Stoics: to live in accord with nature — kata phusin zēn. The phrase is compact and load-bearing. Nature here means several things at once: (1) the rational nature of the human being specifically, (2) the rational order of the cosmos — the logos — and (3) the natural unfolding of one's specific life. To live in accord with nature is to live in accord with all three — to exercise one's rationality, to align with the cosmic order, and to fulfil one's particular role.
What this means in practice is virtue — the four cardinal virtues of practical wisdom, justice, courage, and temperance. The Stoic claim is that virtue is the only genuinely good thing, and the only thing necessary for the flourishing life (eudaimonia). Everything else — health, wealth, reputation, even life itself — is at best a preferred adiaphoron: worth pursuing instrumentally, but not part of what makes a life good.
This is the strongest and most-contested Stoic doctrine. Most people, asked what they need to be happy, list health, financial security, loved ones in good health, meaningful work, decent prospects. The Stoic claim — that virtue alone is sufficient, and all the rest is indifferent — strikes most of us as obviously false. The Stoics' defence is sophisticated and consistent: the sage on the rack is happy because he has not lost what is genuinely his; the prosperous fool is miserable because he has nothing of his own. Most of us do not believe this. The Stoics knew most of us did not believe it. The doctrine stands anyway, as the working hypothesis of the practice — and a Stoicism that drops it has dropped the spine of the system.