Shelley is referring here to the nature of mutability, or change. The central paradox at the heart of the poem is that the one thing that doesn't change is change itself. It's always there, whatever we do, however we feel; happy or sad, rich or poor, change is ever present. Or, as Shelley puts it in the very last line "Nought may endure but Mutability".

In developing his main theme Shelley compels us to look at the fundamental changeability of our nature. According to him, we are like "clouds that veil the midnight moon". If you've ever seen clouds covering the moon you'll know that, before long, they will drift away, leaving the moon to shine brightly.

Like the clouds, we too are restless: constantly changing from one emotional state to another. In our dreams we can have terrible nightmares that poison our sleep. On waking, we can be troubled by a single thought that will ruin our whole day. And yet, these unpleasant states will not last forever. We will weep at times; on other occasions we will laugh. That is because we are mutable.

Whether we experience any kind of emotion, their "path of departure," the manner in which they leave us, is exactly the same as that of any other emotion. In other words, when any emotion leaves us, they do so entirely of their own accord.