Alkanes and alkenes burn in air. In complete combustion there is plenty of oxygen, and the hydrocarbon reacts fully. Every carbon atom ends up in carbon dioxide and every hydrogen atom ends up in water.
hydrocarbon + oxygen → carbon dioxide + water
The hydrocarbon is oxidised because it gains oxygen. Carbon is oxidised to carbon dioxide and hydrogen is oxidised to water. The reaction releases energy as heat, which is why hydrocarbons are used as fuels.
An alkane example is methane: CH4 + 2O2 → CO2 + 2H2O. An alkene example is ethene: C2H4 + 3O2 → 2CO2 + 2H2O. Count the atoms on each side to check that an equation is balanced.