Complex sentence: In politics, crossing the floor has two meanings referring to a change of allegiance in a Westminster system parliament: voting against the approved party lines or changing to a second party after being elected to a first party.
Split into simpler sentences: In politics, crossing the floor is when a politician changes their allegiance or votes against the party in a Westminster system parliament. Crossing the floor may be voting against the approved party lines, or changing to a second party after being elected to a first party.