Complex sentence: On the federal level, it forms a common faction in the "Bundestag" (Federal Parliament) with the CDU, the CDU/CSU faction, which is frequently referred to as "die Unionsfraktion" (the Union faction).
Split into simpler sentences: On the federal level, it forms a common faction in the "Bundestag", the federal parliament, with the CDU. This makes up the CDU/CSU faction, which is frequently referred to as "die Unionsfraktion" (the Union faction).