Highmem is a method that increases the usable address space. Originally developed for Intel i386 where it enables the use of more than approx. 970MB of memory. Most of the code is portable to other architectures.
Memory Managment on Linux/MIPS
32-bit Linux/MIPS kernels assume the entire low memory to be accessible through KSEG0 which limits lowmem to at most 512MB. Typical MIPS systems reserve part of that address space for other uses so a lowmem limit of 256MB is a common value. Memory beyond that can be used by 64-bit kernels or 32-bit kernels with highmem support enabled.