The gross pay is the pay that the employer owes to the employee. The employer deducts the social contributions and taxes (advance tax payment) owed by the employee from this. The remaining amount is the net pay.
If you work with a student contract and you remain within your quota, you will only owe the solidarity contribution for social security. This is much lower than the normal social contribution.
If you only pay the solidarity contribution, in most cases you will not have to pay taxes either. You do have to file a tax return.