In combination reactions, two or more substances (elements or compounds) simply combine to form a new substance.

For example, when a substance burns it combines with oxygen present in the air. 

2Mg (s) + O2 (g)  →  2MgO (s)

C (s) + O2 (g)   → CO2 (g)

CaO (s) + H2O (l) → Ca(OH)2 (aq)

In the above reaction, calcium oxide (quick lime) and water combine and form a single product - calcium hydroxide (slaked lime). Such a reaction in which a single product is formed from two or more reactants is known as combination reaction.

A lot of heat is evolved during the course of the reaction. Such reactions in which heat is released along with the formation of the products are called exothermic reactions. For example, burning of natural gas (CH4) used for cooking:

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)