Symbolism, Imagery, Allegory
Thread of Life
When an emperor who writes his philosophical reflections in Greek brings in the image of a thread or web, there's no way to escape the connection to the most infamous of all mythological thread-bearers: the Moirai, or three Fates. We don't actually have to second-guess this, since Marcus tells us straight up that he's got these ladies on his mind:
Gladly surrender yourself to Clotho: let her spin your thread into whatever web she wills. (4.34)
Marcus is talking about the Fate sister who spins the thread of life for each human being and determines when he or she will be born. She's the one who takes the substance of the universe and turns it into the material of destiny. Pretty cool, huh?
Now, Marcus speaks of Clotho not just as a spinner but also as a weaver. He takes the opportunity to extend the fabric metaphor to suit his own purposes, which is ultimately to talk more about how each human life is connected not only to every other human life, but also to the entire cosmos.
Thread of Destiny
The web is made up of all these "threads of destiny" coming together, creating a community to which everyone is bound—until Atropos (that's sister #3) cuts the thread and ends your participation in the fabric of life. But the substance of the thread really has no ending—just as it has no definite beginning.
That's because all substance—the stuff that creation is made of—is constantly cycling and recycling through the universe. Just as a web of life exists to form a human community, there is another kind of web bringing everything together to create destiny on a macrocosmic level. So, when something unfortunate happens, Marcus tells himself that it's all good, since it's part of a larger pattern:
... the mesh of causes was ever spinning from eternity both of your existence and the incidence of this particular happening. (10.5)
The thread binds us, then, not only to each other, but also to that "mesh of causes"—the impulse that sets creation in motion—in order to continuously create and renew the universe we are all a part of.