The mirror can only reflect the superficial, physical exterior. In Judaism the physical is a vehicle that allows the spiritual dimension to expand and reach up towards God. If a person believes only in the physical, death symbolises the end of everything. However, if a person believes in afterlife, death is the next stage of the ultimate journey. Covering the mirrors symbolises the fact that there is a huge, enormous world beyond the physical world that we live in. It symbolises the fact that this person is no longer with us, but that his spirituality does remain. A reflection in a mirror does not have that extra spiritual dimension, so in order to remove our attention from the purely physical and emphasise the spiritual we cover it.
