Almost universally, separation agreements come from your employer when they are terminating you. Remember that, as they are designed to help them, not you. These agreements, which are contracts, are all about your employer getting what they want.
As such, you should almost never sign them. Honestly, what are they going to do, fire you again or lay you off again? This document helps them, not you.
Typically, the agreement offers some sort of severance pay in exchange for you dropping any potential claims you have against them. While the money may be nice, how do you know if it is a fair amount? You really would not until you sit down with an employment law attorney to go over what potential claims you may have.
Make no doubt that if they are offering you money, it is usually because they think you may have some valid claims against them, even though the document will say otherwise. The fact is, no company is just going to give you money out of the goodness of their heart as they are showing you the door. The slight exception would be if it is a layoff and the company has a set policy on what severance pay is to be.
Instead, tell them you will think about it. If they then start offering more money, you know they are scared. If they threaten not to give you your final paycheck, then let them know they will be violating the law and you would be happy to collect the penalties from them. Then call an attorney.