Real mode denotes a way of operating
Intel microprocessors. It means that only 1MB of memory can be addressed. PC computers execute the first steps in this mode when starting. DOS operates in this mode. The advantage of using the real mode is that the
BIOS routines may be used.
Other more modern operating systems switch to the more powerful
protected mode.
Real mode is the only mode x86 processor designs were capable of operating in until the 80286.
Read on: IA-32, X86 and x86-assembly