It can be theorized that to dream of falling is a physical sensation of a drop on blood pressure. As this registers in the brain, the subconscious will alert you of it in this fashion. It produces a sinking feeling such as this. There is however, a religious side to this concept as well. In Christianity, a realm of eternal suffering is located beneath the surface of the Earth. It is called Hell. It is believed to be home to Demons, or Fallen Angels. These Angels were once in the paradise-like realm among the clouds called Heaven. Greed and jealousy however, turned them cruel and full of hate. This caused them to 'fall' down to the underworld.
Because of this, it is thought by some that to dream of falling represents your fall from purity and suggests that you have strayed from your spiritual path. Perhaps you have cheated someone or misled another. One may have this dream as a sign that they need to check and make sure that their thoughts are noble before they act on them.
But the most widely accepted meanings of falling dreams have nothing to do with science, nor with religion. Generally, if you fall within a dream, it is thought to have to do with balance, or rather a lack thereof. Most likely you have been increasingly stressed due to a recent failure in your life whether it is associated with your job, schooling or a relationship. This fall from glory is dreamt for the purpose of telling the dreamers to get up, dust themselves off and keep on trying.
It is also suspected that to dream of falling stands for your feeling of being below everyone else. The dreamer is belittling or torturing conscience to self-causing a low self-esteem. This type of dream is trying to make them aware of this and show them that they need to have more confidence in themselves and become well rooted so that they will not 'fall' at the first sign of opposition. They need to stand strong.