If you need to add a T2125, it means the slip issuer views you as a self-employed person with “business income” (rather than as an employee with "employment income").
When you’re self-employed, you must report your business income on a T2125. If you didn’t realize you had business, refer to this step-by-step guide for completing the T2125.
Whether someone is an employee or is self-employed can be a very contentious issue. The CRA has a page devoted to the topic here. If you are certain that you are not self-employed, you can report this amount as other employment income on line 10400 or as other income on line 13000. If you choose to do this you must remove the amount from your slip.
It’s your responsibility to properly report your income. Using lines 104 or 130 when the CRA expects you to use the business income section may result in a reassessment.
If you are unsure of what you should do, please call the CRA individual enquiries line.