"Logically distributed" simply means that nodes are capable of sharing their work. No node is ever the only node that can perform a specific task -- other nodes must be able to come in and perform the same function. They can either help (preferred) or take over completely. A more practical and familiar way of looking at this is that the system should have no single point of failure (SPoF).
While this sounds kinda silly, this quality of logical distribution pretty much enables everything else we need for distributed operations.