After being ignited in a bunsen burner flame, a piece of magnesium ribbon burns brightly giving off heat and light in this situation the bunsen burned flame provides

Magnesium reacts with oxygen present in air to produce magnesium oxide;

                                     2 Mg  +  O₂    →     2 MgO

This reaction doesn't start on its own but requires a initiation energy called activation energy. When provided with this activation energy with the help of Bunsen burner it will start a spontaneous reaction because the further energy required for reaction is generated by itself.