A severance agreement allows you to receive pay and benefits upon leaving a job. Whether you want to leave or not, negotiating your agreement is in your best interests.
The U.S. Department of Labor explains there is no federal law requiring a severance package, so any offer by your employer is completely voluntary.
Of course, your employment agreement may include a promise of severance, but even then, the terms usually are up to your employer. This gives you room for negotiation.
When negotiating the terms, you should try to maintain fairness. You should not ask for something you know is not possible or use the package as a way to try to punish your employer for terminating your employment. Instead, you want to think about it in a practical manner.
Consider what your needs will be. Ideally, a severance will help you financially until you can get a new job. In addition, it should reflect the value you provided to the employer. If you were a successful employee, then you should expect compensation to match that.
During negotiations, you should be open to compromise. Remember that unless there is a contractual agreement about the package, your employer is under no legal obligation to provide it to you. The whole process exists between you and them. Becoming combative or difficult could result in a withdrawal of the offer.
At the same time, you should not let your employer take advantage of you. Arm yourself with proof of how you provided value as an employee. That should go a long way towards helping you get a good deal.