This is an insulator for a flask. The shiny part is the thermal insulator and the dark part is the heat sink. Both are critical components of a flasking system. This particular assembly also has attachement slots at both ends.

The insulator is designed so as to minimize the conduction of heat along it's length and therefore, to make it difficult for the heat to penetrate through the open end of the flask. There will be always a certain amount of heat 'leaking' into the flask.

The heat sink is used as a 'storage' component for the heat. The more heat sink mass, the longer it will take to heat up. This is similar as heating an aluminum soda can with a match. If it is empty (light weight) the heat of one single match may be sufficient to melt a hole into it, while if the same is tried with an aluminum cooking pot (heavy weight), the heat of the same match will change the temperature very little. It will require many, many more matches (and time) just to heat it up.