For example, hydrogen burns in the air and reacts with oxygen in the air. This is easy to understand.

There is also a kind of combustion, such as sodium metal burning in water to produce sodium hydroxide and hydrogen. This is actually an oxidation reaction, and sodium is oxidized.

Others call exothermic chemical reactions combustion.

For example, chlorine reacts with hydrogen to produce hydrogen chloride; Quicklime with water, and so on.