It's saying that the operation can be made parallel.

Applying an operation to a total workload is identical to:

1) Split the total workload into smaller workloads.

2) Apply the operation to each smaller workload.

3) Apply the operation to the results from step 2.

Each smaller workload in step 2 can be run in parallel.

While that may seem obvious for adding numbers or money it may be less obvious when you have different operations for example a method in an OO class hierarchy.