This 1972 hit song by Creedence Clearwater Revival is essentially a commentary on how, when it comes to things one ought to know and know about in order to make a fulfilling life out of one's existence, such revelations seem to be the very things that are kept secret from you, that nobody wants to let you in on.
Parents, teachers, and church may freely divulge the basic lessons---morals, math, spelling, social customs and etiquette, accepted historical facts, science---while still keeping the more intimate knowledge among themselves and their close friends and associates
...meaning one has to take it upon themselves to either research or somehow figure out, on their own, certain secrets of the world that most are not willing to share (not only that, but "you'd better learn it fast" and "you'd better learn it young"---before your life completely passes you by).
Or, maybe, the reason a parent might tell their child "Someday you'll understand" is because on-one's ever taught them anything essential either, so they honestly don't know anything themselves, and thus have no secret knowledge of their own to pass on. But, since their kid is so dependent on them as parents to teach them what they don't already know, the parent has to cover up their own ignorance by a "kick-the-can-down-the-road" type of response, sort of trying to creatively give their kid the "runaround" without making it too obvious as to what they're doing.