What do pre-tax and post-tax mean for deductions?

In the context of deductions, pre-tax deductions are taken from an employee's gross earnings before taxes are calculated for those earnings. In other words, the employee's taxable wages are reduced by the amount of the deduction, and the benefit isn't considered taxable.

Post-tax deductions occur after the employee’s gross earnings are taxed. In other words, the amounts that are contributed towards the benefit is considered part of the employee's taxable compensation.