A photon is the smallest piece of light. Light always goes straight, as fast as anything can go, and it hides the space behind it.
If a photon gets curled into a loop it goes around and around, and it keeps hiding the space inside. That trapped light is what we call mass. Everything you can touch is made of trapped light.
Inside (left): the photon runs around and around and hides the space in the middle.
Outside (right): you cannot see the hidden space at all, only a dot. But all the lines of space around it lean in toward it. That leaning is how you can tell something is there, and it is what makes things fall (next page).