Marauder Shields is the fan-name given to a Marauder Husk that appears in the climax of Mass Effect 3, so named because he is the only Marauder in the game to have energy shields. It is also the very last enemy fought in the game.
The character and accompanying meme originated after the extremely controversial ending to the Mass Effect trilogy, which proved to be very divisive among the fanbase due to it's seemingly rushed and contradictory ending that also had little to none of the choice that was such a major part of the series up to that point. As a result people began to joke that the mysterious Marauder with shields, the last enemy the player ever fights, was in fact trying to "save" us from the terrible ending to the game. As a result Marauder Shields is viewed (albeit sarcastically) as a heroic figure despite being a Husk.
The meme has expanded to the point that there is now a comic called Marauder Shields on Deviantart that chronicles in greater details Marauder Shield's efforts and even postulates the idea that Marauder Shields is in fact Nihlus Kryik, thus explaining why he is heroic.