A program global area (PGA) is a memory region that contains data and control information for a server process. It is an area of non-shared memory created by Oracle when a server process is started. Access to the PGA is exclusive to that server process and is read and written only by Oracle code acting on behalf of the process.
The total PGA memory allocated by each server process attached to an Oracle instance is also referred to as the aggregated PGA memory allocated by the instance.
Here the value represents the PGA aggregate target given by initialization parameter PGA_AGGREGATE_TARGET. This is a global target for the Oracle instance, and Oracle tries to ensure that the total amount of PGA memory allocated across all database server processes never exceeds this target.
When PGA_AGGREGATE_TARGET is used, the sizing of work areas for all dedicated sessions is automatic and all *_AREA_SIZE parameters are ignored for these sessions. For shared servers, you still need to control the aggregate PGA memory by using *_AREA_SIZE parameters.