Rb is a tumor suppressor gene that regulates cell division. It prevents a cell from dividing until in has enough proteins to enter the cell cycle by binding to a transcription factor. Rb complexes with E2F, a transcription factor that's needed for a cell to begin mitosis. When the start kinase for entering the cell cycle is activated, Rb is phosphorylated, dissociates with E2F, and the cell enters the S stage.
People born with a defect in an Rb gene have a much higher chance of developing retinoblastoma, a disease where you get icky tumors in your eyes. Since they already have one defective copy of the gene, it only takes one mutation in the second, normal gene for them to develop the disease. In people with two normal copies of the gene, the chances are low that both will develop mutations leading to retinoblastoma.
The Rb gene is also involved in cervical cancer caused by HPV. HPV has genes that integrate into the host genome, and encode proteins that inactivate Rb and p53, which can lead to uncontrolled cell proliferation and possibly cancer.