发横财 is a verb-object compound (离合词) meaning to get rich suddenly, often through luck or dishonest means. The verb 发 ('to make/get') and the object 横财 ('ill-gotten gains') can split: you can insert 了, measure words like 笔, or time words between them, e.g., 发了一笔横财 ('made a fortune'). The tone of hèng (4th) differs from the more common héng (2nd) in 横, and the word carries a slightly negative judgment — it's not a neutral 'strike it rich' but implies the money came too easily or improperly.