Complex sentence: In Australia, a guernsey is a sleeveless woollen shirt worn typically by Australian rules football players and sometimes by rugby players; thus "to get a guernsey" is to be selected for something or to gain recognition for an achievement.
Split into simpler sentences: In Australia, a guernsey is a sleeveless woollen shirt worn sometimes by rugby players. Thus, "to get a guernsey" is to be selected for something or to gain recognition for an achievement.