In Russian "может" expresses ability (physical or mental) or possibility due to circumstances thus corresponding to the English verb CAN

"может быть" expresses supposition with uncertainty and corresponds to the verb MAY

"Он может прийти" He can come (= He is able to come / It is possible for him to come)

"Может быть, он придет" He may come (I suppose he will come but I am not sure)

"Может" и "может быть" in the latter case may be interchangeable when they are used in colloquial speech. In bookish style only "может быть" is a proper choice.