A Combo Card is the combination of an Employment Authorization Document(EAD) and Advance Parole permission (AP). It looks similar to a EAD card itself but include extra text that reads, "Serves as I-512 Advance Parole". Combo card will only be granted when you apply I-131 and I-765 at same time. If you file them separately or you only applied one, you will most likely get a EAD card and/or AP paperwork.
USCIS issues this combo card to people who have a pending adjustment of status petition so the applicant is able to work legally in the U.S. and travel overseas without abandoning the in progress green card application.
How long is combo card valid?
USCIS issue combo card for a period of one or two year depends on the average processing time of your petition. Normally, the validity period should be enough to cover the application pending time. And if your case is taking longer, you can apply to renew your combo card.