Answer: The white matter in the nervous system appears white because it is made up of myelinated neuronal projections.
Myelin is a fatty substance that surrounds the axons of neurons. Myelin is created by oligodendrocytes in the central nervous system, and Schwann cells in the peripheral nervous system. Myelin is 80% lipid, which appears white because light reflects off of it.
Compare white matter to gray matter - gray matter contains more cell bodies than myelinated axons.
The major part of the brain where white matter can be found is the corpus callosum, a bundle of myelinated axons that connect the left and right hemispheres of the brain. Ventral to that, there is another major white matter tract called the anterior commissure, which also sends connections across the two hemispheres.
Try this anatomy book about the brain to learn more about white matter and the corpus callosum.