Severance is the company saying "we'll pay you this much as you leave without you having to work, but just sign this paperwork...". That paperwork boils down to you agreeing this layoff was legal (you're not being fired because of your race or as retaliation for something you did) and you won't take them to court for it. It may also include something like a "non-compete" clause where you agree not to work for a competitor of your ex-employer for some period of time.

Ideally, they'll lay out what is in that severance agreement, and will recommend you get your own legal counsel to read it over. You do not have to sign that paperwork. And if you don't, they don't have to give you anything.

How much severance is offered varies a lot. Often it's based on length of employment. I went through a layoff that gave one week per year of service. Or there may be a base level (say two weeks pay) plus one week severance per two years. It may or may not include continuation of benefits during the length of the severance period (handy for things like health insurance before COBRA kicks in). Or any number of other options. But there's not really anything that you'd definitively be "entitled" to, just what terms are offered and whether or not you accept.

Employees don't really have much leverage around negotiating severance, other than accept / decline. Declining is only going to be beneficial if you think you could win a wrongful termination suit for more than the offered severance (a very long shot in most cases).