 The electrons are generally trapped in localized states (loosely speaking, they are "stuck" to a single atom, and not free to move around the crystal).Occasionally, random thermal fluctuations will give an electron enough energy to get out of its localized state, and move to the conduction band.Once there, the electron can move through the crystal, for a brief amount of time, before relaxing into another localized state (in other words, "sticking" to a different atom).