Epictetus doesn't mess around: the opening paragraph of part 1 of the Enchiridion sets the stage for all Stoic philosophy:
Some things are in our control and others not. Things in our control are opinion, persuit, desire, aversion, and, in a word, whatever are our own actions. Things not in our control are body, property, reputation, command, and, in a word, whatever are not our own actions.
The seperation of things into those that we control and those that we don't is what Irivine calls the Dichotomy of Control, and is the core concept in Stoicism.
In the next paragraph, Epictetus explains why this distinction is important:
The things in our control are by nature free, unrestrained, unhindered; but those not in our control are weak, slavish, restrained, belonging to others. Remember then, that if you suppose that things which are slavish by nature are free, and that what belongs to others is your own, then you will be hindered, you will lament, you will be disturbed, and you will find fault with both gods and men. But if you suppose that only to be your own which is your own, and what belongs to others such as it really is, then no one will ever compel you or restrain you. Further, you will find fault with no one or accuse no one. You will do nothing against your will. No one will hurt you, you will have no enemies, and you will not be harmed.
In other words: by focusing solely on those things which we control (our opinion, persuit, desire, aversion) we can aviod the negative emotions and experiences of life. The rest of the Enchiridion is filled with examples, so I won't flesh this out much more here.
Aiming therefore at such great things, remember that you must not allow yourself to be carried, even with a slight tendency, towards the attainment of lesser things. Instead you must entirey quit some things and for the present postpone the rest. But if you would both have these great things, along with power and riches, then you will not gain even the latter, because you aim at the former too: but you will absolutely fail of the former, by which alone happiness and freedom are achieved.
This is a call to action, and a caution. If you want to achieve the peace and contentment that Stoicism offers, you can't let yourself be distracted by the persuit of other more base desires. As we'll get to later, the Stoics are not anti-wealth, but they are certainly against the desire for wealth.
This is not, as it may appear, a call to head into the wilderness leaving your worldly possessions behind. Instead it is a warning that you cannot aim to simultaneiously improve your Stoic practice and your position in the world. The things to be posponed or removed are particular behaviours and thoughts, not posessions.
Work, therefore, to be able to say to every harsh appearance, "You are but an appearance, and not absolutely the thing you appear to be". And then examine it by those rules which you have, and first, and chiefly, by this: whether it concerns the things which are in our own control, or those which are not; and, if it concerns anything not in our control, be prepared to say that is is nothing to you.
Part 1 closes out, as is often the case, with some practical advice. The first step on the Stoic path is to look at things which upset you, decide if it is something you control, and if not, to let it go.
For example, a collegue might make it clear that they don't respect your work. Do you control what that person things of you? No. So remind yourself that your focus is on your own actions, and move on.