Rich people sometimes look down on people who don't have much money.
Literal
Wealthy people [topic-は] much money [object-を] have-[negative-ていない] people [object-を] look-down-on-[nominal-ことがある].
Two things: 見下す ('to look down on, scorn, despise') is a transitive verb built from 見 (look) + 下す (bring down). The pattern [verb]-ことがある here is the 'sometimes X' frequency pattern — 'there are occasions when X happens.' Distinct from the past ~たことがある ('have done X before'). A critical social observation.