Used mostly in the context of the last step in green card processing (Adjustment of Status), preadjudication means USCIS is proceeding to make decision on the file even though priority date is not yet current. The idea is that way when the PD gets current, all they need to do is send approval notice and then the green card. So if the AOS is preadjudicated, you still need to wait for the PD to become current.