Stoicism has a couple of fundamental ideas: one is that a meaningful life -a life worth living- is a life of practicing virtue. And this is virtue in the Greek/Roman sense of the term.
There are four fundamental virtues:
- Practical wisdom, which is the ability to navigate complex situations in the best possible way.
- Courage, which is not just physical courage, but moral courage, the courage to stand up for situations and for people.
- Temperance, the ability to exercise self control, not to go into excesses of any sorts.
- Justice, which is treating other people with fairness, the way you would like to be treated.
For a Stoic, if you practice sincerely the four virtues, that in and of itself, is both necessary and sufficient to make your life worth living.
The other one is the so called "Dichotomy of Control", best articulated by Epictetus, one of the late Roman Stoics, but present from the beginning of the philosophy, all the way back to Zeno of Citium, its founder, around 300 BC.
Dichotomy of Control says that wisdom and a serene life come out of understanding and internalizing that certain things are under your control and other things are not, and that you should focus on where your agency can actually be effective, that is, on the things that are under your control.
All external happenings are not under your control, you can only influence them. The things that are under your control are your values, your decision and your behaviors, your judgements about things.