The Microsoft Download Center Desktop Heap Monitor Version 8.0 page contains the following overview:
The Desktop Heap Monitor is a tool that examines usage of desktop heap.
WIN32 subsystem has internal heap area known as "desktop heap." When you run a large number of Windows-based programs, "Out Of Memory" error messages appear when you attempt to start new programs or try to use programs that are already running, even though you still have plenty of physical and pagefile memory available.
This behavior can occur if the desktop heap in the WIN32 subsystem is depleted (see KB#126962).
By using the Desktop Heap Monitor, you can examine how much desktop heap is consumed without interrupting the running system.