While the main story of Mass Effect 2 focuses on the threat of the Collectors, a mysterious bug-like species who have been abducting human colonies, a massive portion of the game is spent completing missions that relate to Shephard's squadmates. These are called loyalty missions, and they're extremely important to play through since they can decide whether or not a certain character dies during the final Suicide Mission, so for anyone hoping to keep everyone alive until the end, it's necessary to finish each one of them.