A severance agreement usually pays you in exchange for giving up claims against your employer. Read the release first: it says which claims you give up (sample note).
Then check the payment: how much, when, and whether it includes holiday pay you are owed anyway. Look for confidentiality, reference and non-compete clauses, and the date the offer expires.
Many agreements give you time to take advice. This note is general information; the terms that apply to you depend on your contract and the law of your state.