A precursor is a compound which the body metabolizes to an active compound. For example, DHEA, 4-AD (the original 4-androstenediol, not a recent compound given the same abbreviation) and androstenedione were precursors to testosterone.
The precursors, except for DHEA, have been banned by the government, as well as nearly all the various other active compounds -- not precursors -- that were the best choices among the non-pharmaceutical anabolic steroids.
The stuff sold today is the leftovers that weren't worth being pharmaceuticals and weren't worth being sold as "supplements" prior to the ban, and therefore didn't get named in the ban.
For good anabolic steroid type results, pharmaceutical anabolic steroids are the way to go, not the OTC junk. In many cases the side effects of the OTC products are worse, and almost invariably the balance of gains to adverse side effects is worse.