Variable costs are costs that change with the production volume. If the level of production decreases, total variable costs decrease as well and vice versa. But unit variable costs remain unchanged with the changed production volume.
Opposite to variable costs are fixed costs.
Variable costs do not necessarily need to be direct (e.g. variable component of power costs for operating production lines which vary with the production level, but it is difficult to clearly identify it with a product) and direct costs do not necessarily need to be variable (e.g. costs of setting-up a production line for a specific product where number of units produced are irrelevant).