In chess, an opening system is an opening that can be played regardless of what moves the opponent plays. Most chess openings require both white and black to make a precise series of moves to be achieved. In contrast, opening systems aim at achieving a specific piece setup without attending much to an opponent's play.
Examples of system openings include the London System, the Colle System, the Cow, and the Hippopotamus Defense.
System openings have several possible advantages over regular openings, including no need to be familiar with multiple different openings to account for the opponent's possible moves, a potentially much deeper familiarity with various lines arising from only having to memorize a single opening, saving time on the clock by not having to think too much for the first several moves, and having a fairly iron-clad "guarantee" of reaching a position the player knows and is comfortable with out of the opening.
Possible disadvantages include a lack of flexibility to aggressively attack the opponent's weaknesses, excessive predictability when facing the same opponent multiple times, and difficulty dealing with an opponent who is prepared to neutralize or defeat that specific system.