The word "laser" stands for "light amplification by stimulated emission of radiation." The tissue effect of lasers is characterized by understanding the absorption of light in tissue. The three main components of tissue that affect the absorption of light are water, haemoglobin (pigment that renders blood red) and melanin (pigment that gives skin its natural color). The absorption curves for these three substances versus the laser wavelength will determine the precise impact that a particular laser will have on tissue.
The purpose of a low level laser is to stimulate. The lower energy levels and the unfocused light beam do not impart large amounts of energy. They do provide enough energy to excite the mitochondria and cause it to undertake big-chemical reactions.