This almost always refers to a metal alloy consisting mostly of zinc, but also with about 4% aluminum, and trace amounts of magnesium, copper, lead, iron, cadmium, and tin. The trade name for the alloy is "ZnDC", referring to "zinc for die casting".
The name "diecast metal" is a colloquial phrase, referring more to the manner in which the metal is molded or cast than the material itself. In all likelihood, some metal used in toys that's called "diecast" was probably formed using a means other than being cast in a die, but diecast is the name that's stuck over time.
Toys containing Diecast Metal
Articles on Diecast Metal toys