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 Handover.
Now coming to your second part Why is Inter-RAT is done
Reason is simple if word is uniform i.e. everywhere ALL LTE then it is not required so to provide the handover from one network technology to another one InterRAT mobility is required.