Globally Unique Temporary Identifier or with shortened version GUTI is allocated to a UE by the MME when the UE attaches to the Network. And it is used instead of the IMSI to identify the UE. When the UE attaches the network as first time IMSI is used, thereafter GUTI is used to avoid any security issues in air interface. GUTI format is a little bit complex and consists of other identifiers. The value of GUTI is temporary unlike IMSI.
GUTI is a complex combination of GUMMEI, M-TMSI, S-TMSI, MMEC, MMEGI, MNC, MCC, PLMN and MMEI.
General overview is more clear on the above image.