Ancestral Knowledge is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Cumulative upkeep (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.) When this enchantment enters, look at the top ten cards of your library, then exile any number of them and put the rest back on top of your library in any order. When this enchantment leaves the battlefield, shuffle your library.