Garnishee orders are granted by the court if you fail to repay a loan, hire-purchase or a payment you promised to make (like a maintenance payment).
When the court grants a garnishee order, this is brought to the employer so that they can deduct the money from your salary. Your employer has to provide you with a copy of the order.
If you do not agree with the order, you need to go to court to appeal against it. But this can be costly and expensive in terms of time and money and this is why you should be EXTREMELY careful when signing any contract for a loan or credit.