The transporter platform (or transporter pad) was a component of the transporter system, where the individual or object that was being transported had to be placed in order to be successfully transported. Some transporter systems supported site-to-site transport and did not require the use of the transporter pad at all.
Personnel transporter platforms often consisted of multiple individual pads sized approximately one per person. Early 23rd century Federation transporter platforms typically had six pads in a hexagonal arrangement; by the latter end of that century they were generally placed in a circular arrangement with a larger pad in the center for bulky items, an arrangement that continued to be used into the 24th century. (Star Trek: The Original Series; Star Trek V: The Final Frontier; Star Trek: The Next Generation) Notable exceptions to this include the early transporter models of the 22nd century, which had just one largish circular pad. (Star Trek: Enterprise)
Each pad in a Federation transporter had four redundant molecular imaging scanners so that if one of the scanners failed, the other three took over. When necessary, the entire platform could be enclosed within a force field. (TNG: "Realm of Fear")