A monitor is usually a small program that acts as a mini-executive to control a computer. An RTOS may have a monitor but an RTOS is not a monitor.

The purpose of a monitor is to allow code and data transfer and a simple run-time environment for that dowloaded as code.

Typically, you can download and upload to the target using simple command and equally simple data exchange (i.e. intel or "S" Hex File formats and sometimes binary formats (X-modem, DDCMP, etc).

Also some format for code and data review is provided (Hex Dumps with ASCII decode sidebar being very typical).

Finally some I/O control may be provided.