A REDEFINES allows a different data description for storage which has already been defined.
That might be to allow entirely different data to be held there.
05 RL-AGENT-DATA.
...
05 RL-CUSTOMER-DATA REDEFINES RL-AGENT-DATA.
...
Or to give a description which allows a use of a part of the data, as in Bruce's example.
Or to use a piece of data that is the same, but for a different purpose:
05 INPUT-AMOUNT PIC X(10).
05 INPUT-AMOUNT-NUMERIC REDEFINES PIC 9(8)V99.
Whatever is in INPUT-AMOUNT it can be reported without problem, but only if it is a valid numeric (by testng it for NUMERIC), is INPUT-AMOUNT-NUMERIC used for some purpose.
Note that each 01 subsequent to the first under an FD is an "implicit REDEFINES".
Note also that items in the LINKAGE SECTION have the effect of "redefining" "something", even though if the address of the data is from a CALLing program, the definition is often the same as the original definition, and should usually at least match the PICtures of the original.