The High Memory Area (HMA) is the first 64 KByte after the first 1024 Kbyte (Conventional+UMA).

Memory	Size	Provider
XMS/EMS	? K	Himem.sys/Emm386.exe
HMA	64K	Himem.sys
UMA/UMB	384K	Emm386.exe
Conventional	640K

It is possible to load some of the DOS environment into the HMA and free up some conventional memory.

Edit CONFIG.SYS and update/insert these lines:

DEVICE=C:\DOS\HIMEM.SYS
DOS=HIGH

Note the HIMEM.SYS is a driver that forfills the eXtended Memory Specification (XMS), which specifies how to allocate memory beyond the first 1024 KByte and therefore also the HMA. The memory beyond the HMA is called extended memory or just XMS.

More Info MS KB91088
More Info MS KB72302
More Info MS KB96711
More Info MS KB105557