According to Epictetus, all we really have control over are our judgements, along with things that derive from our judgements. We don’t have complete control over everything in our minds, we don’t choose the sensations and memories that we have, and we cannot switch on and off our emotions.
No, all we have complete control over are our judgements, which is to say what we think about the things that happen to us.
Now, our judgements are hugely important because, among other things, they determine how we act. As Epictetus put it, they control our desires and impulses. We might see something, make a judgement that it is something good, which creates a desire for it, which in turn prompts us to pursue it. Depending on what the thing is - a dream career, an expensive house - it might be a long and arduous pursuit, carried out at great cost to both ourselves and others. But the whole process begins with a simple act of judgement.
So, judgements are fundamental, and we neglect them at our own peril, but we often make them so swiftly that we don’t even notice that we are doing anything.