This metal disc is an alloy that melts at a relatively low temperature; in this case, around 200 °F (95 °C), which is lower than the temperature of boiling water.
Fusible metals are alloys that melt at low temperatures, generally below 300 °F (149 °C), and are commonly used by hobbyists since they can be used to make custom jigs, tools, and casts in situations that would not be cost-effective for mass production. Plus, once a project is complete, it can be remelted and used for a new project.
The low melting point is caused by the addition of the elements bismuth and tin to the alloy composition, both of which have somewhat low melting points.
Fusible metals are also used in automatic fire sprinklers common in commercial buildings. Water pressure in the sprinkler pipe is held in by a plug made of an alloy designed to melt at a specific temperature. When a fire is present, the heat melts the plug, and the pressure forces the water out.
This alloy contains some lead. Do not ingest it.