Maximum addressable memory is the most RAM a CPU is able to talk to. The CPU manufacturer determines this when they design the CPU.

The older CPUs could only talk to a small amount of RAM. As technology progressed, manufacturers developed CPUs that could address more and more RAM. It is hard to believe the first IBM-compatible CPU, the Intel 8088, could only talk to 1 MB of RAM. Today's CPUs can talk to, or address, 64 GB or more! This is quite a technological advance over the years!