The x-ray grid was first used by Gustave Bucky in 1913 to reduce the amount of scatter reaching the image receptor. The grid was stationary, which caused grid lines on the films, but the reduction in the amount of scatter reaching the film greatly improved the contrast of the radiograph.
The grid is made using strips of lead sandwiched between radiolucent material. The lead strips are angled in such a way as to only allow x-rays from the primary beam to pass through to the image receptor. The grid is placed between the patient and the image receptor.
These days the grid is built into the table and upright Bucky system. However there are also stationary grids, that are either built into the cassette or encased in a radiolucent material, available for use on bed-ridden patients and for use in trauma radiology.