Salary sacrificing is an arrangement where you receive part of your gross salary as a benefit rather than salary. The benefit is paid from your gross salary, i.e. before tax.
Salary sacrificing to make super contributions will reduce your take-home pay but because it reduces your taxable income, you may pay less tax overall. Your super contribution is taxed at 15% when it is contributed to your fund but for most people this rate will be lower than their marginal tax rate that would have paid if the contribution were received as salary.
Note that your salary sacrifice contributions will count towards your concessional contributions cap.