A metal gate, in the context of a lateral Metal-Oxide-Semiconductor MOS stack, is just that—the gate material is made from a metal.
For decades, the industry had moved away from metal as the gate material in the MOS stack due to fabrication complications. A material called polysilicon (silicon, highly doped with donors or acceptors, polycrystalline) was used instead because it can be deposited easily and is tolerant to subsequent manufacturing steps which involve extremely high temperatures (in excess of 900-1000 °C), where metal was not. Polysilicon is also attractive for the easy manufacturing of self-aligned gates. The implantation of source and drain is carried out with the gate in place, leading to a channel perfectly aligned to the gate without more lithographic steps. Furthermore, metal has a tendency to disperse into silicon during these thermal annealing steps.
However, polysilicon doesn't offer the near-zero electrical resistance of metals, and is therefore not ideal for charging and discharging the gate capacitance of the transistor.
From the 45 nm node onward, the metal gate technology returns, together with the use of high-k materials, pioneered by Intel announcements.