Some companies offer loans to employees against their future earnings. Generally, these kind of loans will not carry any interest.
The tenure, number of installments, etc. would be as per the terms of the company. Typically, the installment amount is deducted from subsequent monthly salaries of the employee. However, if the employee resigns before completing the loan repayment, the company may request employee to settle immediately or adjust in Full & Final Settlement as part of dues clearance.