When an agent or an editor requests a reader’s report on a manuscript which has been submitted to them, they’ll usually send those reports onto the writer once they’ve made a decision about the manuscript concerned, whatever that decision is—and whatever the reader has reported.
Reader’s reports often contain some strong criticism and discuss the manuscript in very frank terms, and if you’re not able to deal well with constructive criticism they can be very painful to read. But if you’re keen to make your work the best it can possibly be, and can overcome the initial disappointment and hurt that you might feel, these reports can be extremely helpful: they focus closely on a manuscript’s flaws and so make very clear how it could be improved.
Reader’s reports are very valuable tools for a writer who is serious about improving his craft, but agents and editors only ever request reader’s reports on a tiny proportion of submissions: the ones which show real promise. And as most submissions are rejected a long way before a reader’s report is even considered, relatively few writers will ever get to see one.