This is "surgical" terminology to describe the timing of breast reconstruction in either the "immediate" situation after mastectomy, or in a "delayed" fashion, after many months or years after the mastectomy. So the key difference is months, not hours.
There are advantages and disadvantages to each "timing" method. First, the advantage of "immediate" reconstruction is that breast reconstruction and mastectomy are performed in 1 operation. One does not have to wait for the "healing" after the mastectomy (typically 3-6 months), and wait for another operation to reconstruct the breast. In terms of aesthetics, if a "skin sparing" mastectomy can be performed, the breast shape can be excellent and the overall result of the breast reconstruction can be superb. The "skin sparing" techninque allows the surgeon to use the remaining skin as a cover over the flap (DIEP, SIEA, TRAM) or implant/tissue expander that will help shape the breast into a more natural mound.
The disadvantage of the "immediate" technique lies with the potential oncologic issues that can occur if the cancer issues are not fully eradicated, and the breast has been reconstructed with the possibility that cancer cells may come back to recur in the reconstructed breast. The other concern is radiation therapy. In the immediate reconstruction, radiation can adversely affect the outcome of the breast reconstruction (shape, healing, capsular contracture, etc).
The "delayed" technique is still the safest. It provides the optimum time for healing after mastectomy, addresses issues of "recurrence," and allows the operation to be split in 2 stages allowing the patient to recover from the initial mastectomy and then address the reconstruction later. The cosmetic result can still be excellent, with the "immediate" reconstruction having a slight advantage in overall shape.
The choice of "immediate" versus "delayed" is not an easy one. The overall team that is available is critical in choosing either method. This includes the general surgeons, plastic surgeons, oncologists, and nurses.