Moodle can display "recent activity" on the course home page.
This shows the everything that has happened in the course since the last time the viewer logged in to the course, including new posts, new users etc.
It is highly recommended you leave this feature enabled in your courses, as it helps give a sense of activity in the course. Knowing what everyone else is doing also helps to promote a collaborative atmosphere in the class.
You might want to disable this feature for extremely large or heavily used courses, as it may slow down the display of the course page.