In affluent societies, most people maintain a high standard of living.
Literal
Affluent society [in-では], most [possessive-の] people [topic-が] high living standard [object-を] maintaining-[ongoing-ている]
で+は topicalizes the location ('as for in affluent societies'). 豊か is a na-adjective meaning rich, abundant, affluent. 生活水準 is a compound noun: 生活 (daily life) + 水準 (level/standard). ~ている marks the ongoing state of maintaining.