The d4 is a platonic solid known as a tetrahedron. It is sometimes colloquially referred to as a caltrop due to its standard, viciously pointy shape, but can also be referred to as a pyramid die.
A standard, triangular d4 will have three numbers on each of its four faces. Most d4s will display a number near each corner. The result of your roll is therefore the number seated at the topmost corner. Older d4s, including the first d4s ever made for D&D, had their numbers centered along each edge of a die face. On these d4s, the result of any given roll was seated along the bottom of the die rather than at the top.
The d4 is among the oldest dice ever discovered in an archeological dig. The Royal Game of Ur used d4s. One such d4 was discovered in an ancient dig site and dated to be over 5,000 years old.