vSphere ESXi Hosts use NUMA microprocessor architecture, each CPU socket has its own memory. We have interconnection between the sockets allowing CPUs in one socket to access memory that is attached to the other CPU memory bus but this interconnect result in VM slow down. A NUMA Node is a processor and its local memory. Memory and cores accessed outside the NUMA node will have latency and Core or memory local to the NUMA node is always faster. Memory controller connects to Memory modules what we call us DIMMS.

The ultimate explanation can be found here: http://frankdenneman.nl/2016/07/06/introduction-2016-numa-deep-dive-series/