Variable costs are costs that fluctuate in direct proportion to the volume of units produced. The best and most obvious example are physical costs of goods sold, direct costs, such as materials, products purchased for resale, production costs and overhead, etc.
The concept of variable cost is an important component of risk in a company, because generally variable costs are less risky than fixed costs, because variable costs are not incurred unless there are sales and production. See also break-even analysis, fixed costs, and contribution.
For more on this, check out What Is Break-Even Analysis? and Understanding Fixed and Variable Costs and Burn Rate.