ShortMAC-I is the 16 least significant bits of calculated MAC-I. Usually when a RRC message is exchanged between UE and eNodeB it is integrity & ciphered protected. This protected message is transmitted over DCCH (Dedicated control channel).
In this particular RRC re-establishment, message is sent from UE to eNodeB over CCCH (common control channel). This common control channel is neither integrity protected nor the ciphered.
RRC at eNodeB receives un-protected and un-ciphered RRC Re-establishment Request message. After receiving this message, eNodeB first gets UE context from the received "Re-establish UE Identity" and then does verification of short-MAC-I.
MAC-I is calculated by using the stored Key RRCint, VarShortMAC-I message. In formula it can be written like
MAC- I = f(key, varShortMAC-I, count, bearer, direction). Here varShortMAC-I is prepared by using (current cellId, pci of source cell and C-RNTI of old cell). Other parameters like count, bearer and direction set to all 1.