Answer is: There are more reactants and one product of the reaction. The reactants may be elements or compounds. The product is always a compound.
Synthesis reaction is a type of reaction in which multiple reactants combine to form a single product.
For example, balanced chemical reaction: Ba + F₂ → BaF₂.
In barium fluoride, barium has oxidation number +2 and fluorine has oxidation number -1, so compound has neutral charge.
Second chemical reaction: CaO(s) + H₂O(l) → Ca(OH)₂(aq).
One molecule is produced from two molecules.