Everything we see and touch, all matter and energy, is actually made of incredibly tiny loops or strands of vibrating energy.
The way these “strings” vibrate, their shape, and how they interact with each other, are ultimately responsible for the behavior of all of the fundamental particles in quantum mechanics. For example, the particle called a photon—the thing that is responsible for light—is actually a string of energy. It’s a photon—as opposed to an electron—because of the shape and behavior of the string.
In order for the theory to work, the strings do not just move through three spatial dimensions. There are also 6 or 7 (I forgot how many) other dimensions. These dimensions are tiny and “folded up,” so we don’t experience them on the scale we live our lives. But the strings are so small that they actually fall into the folds of the extra dimensions. The extra dimensions are necessary because they help determine the shape and behavior of the strings, which would apparently make no mathematical sense without them.
Another neat idea that goes along with string theory is the “holographic principle.” This is the idea that the universe is fundamentally made of information. A string can be described by only a few bits of information. Matter and energy are thus incidentals of more fundamental information. The holographic principle gets its name from the idea that our 3-D experience of the universe is like a “hologram” that gets projected from a more fundamental, 2-D “shell.” We know this can happen because you can describe everything about the 3-D structure of a black hole simply by looking at the information on its 2-D surface.