Here is an experiment you can do right now. Touch your left hand with your right. Notice what happens: you are simultaneously touching and being touched. Your right hand feels the left; your left hand feels the right. You are, at the exact same moment, subject (the one who feels) and object (the one who is felt). You can flip between these two perspectives, attend to what your right hand is feeling, then shift to what your left hand feels, but you can't quite inhabit both simultaneously. There's a flicker, a crossing.
Merleau-Ponty spent years working out the philosophical implications of this simple experience, culminating in his unfinished masterpiece The Visible and the Invisible (1968, posthumous). His conclusion: this reversibility, the possibility that any sensing being can also be sensed, that the toucher can become the touched, is not a quirk of hands. It is the fundamental structure of all perception.
When you look at a painting, you are also visible to others. When you listen to music, you are also a body that makes sound. You are never a pure observer standing outside the world, pointing a camera at it from a safe distance. You are always already part of the world you perceive, made of the same stuff, implicated in it, caught in its web of relations.
Merleau-Ponty calls this shared stuff the flesh (la chair du monde, "the flesh of the world"). This is his most abstract concept and also, I think, the most genuinely strange and beautiful idea in his philosophy. The flesh is not biological tissue. It is his word for the primordial medium from which both perceiver and perceived emerge, the underlying fabric that makes their interaction possible because they are, at bottom, made of the same material.
The best analogy: think of a length of folded cloth. The cloth has two surfaces, top and bottom, but they are not two separate objects. They are aspects of one continuous thing. Consciousness and world are like this: not two separate substances (mind and matter) that somehow get connected, but two surfaces of one fold in the flesh of reality. The world "sees itself" through you; you are the place where the flesh of the world becomes locally self-aware.
The body is our general medium for having a world. Sometimes it is restricted to the actions necessary for the conservation of life, and accordingly it posits around us a biological world; at other times, elaborating upon these primary actions and moving from their literal to a figurative meaning, it manifests through them a core of new significance.
— Merleau-Ponty, Phenomenology of Perception (1945); quoted in Sensualanimist (2012)
This is neither idealism (the world is made of mind) nor reductive materialism (consciousness is just matter). It is an attempt to describe a level of reality prior to the split between subject and object, a level at which that split hasn't yet happened. Think of how hard it is to say whether a song is in the air or in your head while you're moved by it. That difficulty is not confusion, it is pointing at something real: the song is a shared event between you and the acoustic world. Flesh is the ontological name for the medium in which such shared events are possible.