This is for the I-9 form- for immigration purposes. If you are hired, you will need to provide this proof- US Passport or Drivers License and Social Security Card are a few "proofs" that are commonly shown. Good luck! Hope you get the job!
References :

to work you must complete an I-9 form which requires identification and proof of Employment Eligibility.
if you have a passport that is all you need
if you do not, then you must have an ID and the proof of Employment Eligibility which can be a social security card or birth certificate.

there are numerous documents that can be used as identification and a few different forms of proof of Employment Eligibility but the common ones have been listed.
this link shows all the different types of documents you can use:
http://www.uscis.gov/files/form/I-9.pdf

you are correct this has nothing to do with a work permit.
References :

It is ILLEGAL for an employer to specify what type of identification you must provide. The instructions on what constitutes legal proof of your availability to work in the United States can be found in the instructions for the I-9, which should be provided to you along with the form. Your employer has 3 days from your date of hire to complete the I-9.

You must provide either 1 document from List A (establishes both identity and employment eligibility), or 1 document from List B (established identity) AND List C (established employment eligibility).

Examples of documents that establish your eligibility to work in the US (from the most recent (2008) version of the I-9):
- US Passport (expired or unexpired)
- US Social Security Card)
- Permanent Resident Card
- Unexpired Foreign Passport with temporary I-551 stamp
- Original or certified copy of Birth Certificate
- US Citizen ID Card
- Native American tribal document
- Unexpired employment authorization card issued by DHS
References :
http://www.jwbcpas.com/images/2008_I-9.pdf