The activity log file contains all activities that have occurred during a sequencer run.
Uncheck the Auto scroll checkbox to not auto scroll activities, when checked the list box will scroll down automatically when a new activity will be added.
The activities will help you understand what is happening during a sequencer run, it also shows potentials problems when they occur.
Example activity log
The example log shows all activities that can occur during a sequencer run, note the ERROR part at the end.
You can clear the log trough the "Tools" menu within the IM Sequencer, this option is only available when there is something within the activity log.