Our body is composed of trillions of cells. Some cells, skin for example, divide frequently, replacing those being sloughed off the surface. Some cells, such as nerve cells, may never divide once they have been produced. There must be something that controls the rate at which these cells divide. If a mutation causes a change in the gene, that amount of protein produced is decreased and the division of the cell is no longer controlled.
Cancer is the result.
Remember, there are two copies of each gene in the cell. For the tumor repressor gene, as long as one gene functions normally, the cell will remain under control. Therefore the mutated form of the gene is recessive. Every cell in that individual's body has only one good tumor suppresser gene. If something in the environment, such as UV radiation, smoke, or chemicals damages the remaining tumor suppresser gene on one cell, that cell will no longer have its control mechanism and a cancer may develop.