Similar to an F-Stop, some lenses, particularly zoom lenses, will have f-stops on one side of the aperture ring and t-stops on the other. To differentiate the two, the t-stops will be red and the f-stops white. T-stops are used in place of f-stops for setting exposure. Lenses with a lot of glass elements will often lose a little bit of light. The t-stops are calibrated to the actual amount of light that is hitting the film, rather than arrived at mathematically, as is the case with f-stops. However, the f-stops are still relevant, because while the t-stop should be used to set the exposure, the resulting f-stop will indicate how much Depth of Field you have.
Like F-number, measures the diameter of the lens opening. T-stop factors in the loss of light through the glass lens elements.