What does "Cycle \$\mu\$s" mean in this context?
Ferrite core memory has to be read and then rewritten to retrieve data as the read operation is destructive.

The time to do this read followed by a write is referred to as the cycle time and is typically in the region of one microsecond to a few microseconds as shown in the right-hand column.

The Greek letter mu is the symbol shown to represent micro.