If you mean that they can see all the courses they could enrol in,then that is the normal way if you have your courses displayed on the front page in the central area, If you only want them to see the courses they are enrolled in then one way to do this is to turn on "my Moodle" which you can do in site administration>appearance>my Moodle. When they are logged in it takes them to a page which just shows them their own courses and not the others. Then you would need to set your front page so that courses were not shown when users are not logged in/or logged in, whichever.
Lesson
hide courses
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