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A synthesis reaction, also known as a combination reaction, is characterized by the formation of new chemical compounds through the combination of two or more simple substances. In this process, reactants interact to create more complex molecules, which involve the formation of chemical bonds. For instance, when hydrogen gas reacts with oxygen gas, water is formed as a product. This illustrates how a synthesis reaction leads to the formation of new substances as chemical bonds are created between the constituent elements.

The other options do not accurately describe a synthesis reaction. Breaking chemical bonds pertains to decomposition reactions, where compounds are broken down into simpler substances. Separating elements implies a process where a compound is divided into its individual parts, which also relates to decomposition. Producing energy without reactants does not align with the definition or nature of synthesis reactions, as they typically involve specific reactants and energy changes depending on the process. Thus, the correct understanding of a synthesis reaction involves recognizing its role in forming chemical bonds between reactants, resulting in new compounds.