NUMA stands for non-uniform memory access. It is a memory architecture for multiple CPU systems where some memory is local to specific CPUs, and accessing remote memory (which is local to another CPU) is slower. This is commonly found in 8-CPU systems.
Apparently many databases have problems using NUMA.
Oracle has built-in support for NUMA servers. However it comes at a cost. Higher CPU utilisation and slower file system utilities. So by default, it is disabled in Oracle databases. This is explained in gory detail by Kevin Closson.