A change from one substance to another is called a chemical change or a chemical reaction.  A chemical equation is a statement of a chemical change using chemical symbols.  For example, when one of methane's hydrogen atoms is replaced by a second carbon atom, a chemical reaction occurs and ethane and hydrogen are formed.  This reaction can be expressed as a chemical equation.

Notice that an equal number of each type of atoms exists on each side of the equation.

This is consistent with the Law of Conservation of Energy you studied in Unit 1, Section 1.  Chemical equations are meaningless unless they are balanced.

CH₄ + CH₄ --> C₂H₆ + H₂
The arrow means "yields" or "changes into."