The 2 GB limit refers to a physical memory barrier for a process running on a 32-bit operating system, which can only use a maximum of 2 GB of memory. The problem mainly affects 32-bit versions of operating systems like Microsoft Windows and Linux, although some variants of the latter can overcome this barrier. It is also found in servers like FTP servers or embedded systems like Xbox. The use of Physical Address Extension (PAE) can help overcome this barrier.
32-bit processes running on 64-bit Windows are also limited to 2 GB by default, but executables can have a "large address aware" flag set to increase this to 4 GB.
It could also refer to the maximum partition size for the FAT16 file system in versions of Windows prior to XP.
References
2 GB limit Wikipedia(Text) CC BY-SA