Hi
InterRAT mobility mean to be UE is moving from one cell to another cell by changing its Radio Access Technology..
consider that a UE is moving form Cell A to Cell B. Assume Cell A is LTE cell and in Cell B is an UMTS cell, if handover happens from cell A to Cell B then this kind of handover is called as InterRAT or Inter 3GPP handover,
if UE is in idle state and moving from CELL A to CELL B then the mobility management procedure will happen in Target Cell so this is called as InterRAT mobility.
Thanks,
Srinivas.