As Preben mentioned, it depends on your enrollment method. It also depends upon what you what as "visible but not available."
For example, if you go to my Moodle you will see my courses. You can even get some "information." But you cannot long in without a course enrollment key, which I email to students when I am ready for them to begin (actually, today!) You can also set a start date for an enrollment method. Oh, my process is using "self-enrollment." I can't help you with other methods because self-enrollment is all that I ever use.
For example, if you go to my Moodle you will see my courses. You can even get some "information." But you cannot long in without a course enrollment key, which I email to students when I am ready for them to begin (actually, today!) You can also set a start date for an enrollment method. Oh, my process is using "self-enrollment." I can't help you with other methods because self-enrollment is all that I ever use.
If you mean "a course is fully visible in Moodle, but not available" then this gets a little trickier. I don't do this myself, but others here on Moodle.org might guide you. You could, for example, mess with "guest access" (which I don't allow.)
I run a small Moodle, so you won't see a thousand courses, only a few.