During a total lunar eclipse, the moon is cut off from its light supply (the sun) because the earth moves between the two.
Part of the reason the moon looks red is a phenomenon known as Rayleigh scattering, which is the element that causes the colors of the sunrise and sunset.
Another reason is the atmosphere and the particles in the atmosphere. When the sunlight strikes these particles, different particles with different colors of different wavelengths scatter. The colors with longer wavelengths (like red) are able to pass through the atmosphere down to earth so that we see red.
