When you receive a severance offer, it usually has a date for you to respond. Often it is 21 days, sometimes longer, sometimes shorter.
Legally, it is immaterial whether you respond by the deadline or not. The employer can retract its offer anytime before the deadline expires anyway, so all the deadline means is that they are telling you they do not promise the offer will be on the table past that date.
Practically, it can matter. If your goal is to negotiate a better severance, an employer may take the position if the deadline has passed that it no longer is willing to entertain severance.
This is why it is important to seek legal advice as soon as you know you have a severance offer to consider. You may think you have legal claims, but find out that they are either not viable or not worth pursuing. In that case, you do not want to let the offer expire and walk away from some compensation that could help ease your job transition.
On the flip side, if you do have strong claims but still want to negotiate an adequate severance, the more time you give your attorney to assess and articulate those claims before the deadline, the better your chances of a positive outcome.