Lieutenant Nyota Uhura 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

Strange new worlds — , : Target opponent exiles the top X cards of their library, where X is the number of differently named lands you control. You may cast an instant or sorcery spell from among them without paying its mana cost. Then that player puts the rest on the bottom of their library in a random order.