A monitor is a unit of observation of something, usually a system log, like Q2.log, or a log DataBase table or a transactional table.
Here is the model.
An example monitor is:
<!DOCTYPE entity SYSTEM "cfg/monitor.dtd">
<monitor>
<id>q2</id>
<source class="org.jpos.ee.pm.core.monitor.FileMonitorSource" >
<properties>
<property name="filename" value="log/q2.log" />
</properties>
</source>
<formatter class="org.jpos.ee.pm.core.monitor.MonitorFormatter" />
<delay>3000</delay>
<max>100</max>
</monitor>
The result of monitoring this file is something like
It's an auto-refresh window that shows the tail of the file.