Magnesium is a reactive metal. It is a soft metal and available in a lab as a thin ribbon for laboratory experiments.
When the ribbon comes into direct contact with air, its outer layer forms its oxide.
Magnesium oxide is a stable compound and interferes with burning.
2Mg(s) + O2(g) ——> 2MgO(s)
Cleaning of the ribbon scrapes out the outer layer. Now the ribbon is ready for further experiments like burning.