If you mean the
terminal monitor command, that will send debug and system messages to the current terminal session. If you are connected to the device via a VTY line, you need to use this command to see those because it may look like you are connected the the CON line, but you are not.
Cisco maintains extensive documentation that is just a search away:
Cisco IOS Configuration Fundamentals Command Reference, Release 12.2:
To display debug command output and system error messages for the
current terminal and session, use the terminal monitor EXEC
This command has no arguments or keywords.
Remember that all terminal parameter-setting commands are set locally
and do not remain in effect after a session is ended.
In the following example the system is configured to display debug
command output and error messages during the current terminal session:
Router# terminal monitor
logging montor command will enable terminal line (monitor) logging.
Cisco IOS Network Management Command Reference
To enable system message logging to the terminal lines (monitor
connections), use the logging monitor command in global
configuration mode. To disable logging to terminal lines other than
the console line, use the no form of this command.
logging monitor [discriminator discr-name] [severity-level]
no logging monitor
The logging monitor function is disabled.
Global configuration (config)
Specifying a severity-level causes messages both at that level and at
numerically lower levels to be displayed to the monitor. Table 32
shows a list of levels and corresponding syslog definitions.
Table 32 Error Message Logging Priorities and Corresponding Syslog
The following example shows how to specify that messages at levels 3
(errors), 2 (critical), 1 (alerts), and 0 (emergencies) be logged to
Router(config)# logging monitor 3
The following example shows how to use a discriminator named monitor1
to filter critical messages, meaning that messages at levels 0, 1, and
2 are filtered:
Router(config)# logging monitor discriminator monitor1 critical