By the 19th century London was the busiest point in the world, and this became the main source of employment in the East End. Those who could afford to live in more present surroundings moved out, and the area become one where the vast majority of people lived in extreme poverty and suffered from appealing sanitary conditions.

这句话应该很好懂。move out = moved out the East End（从伦敦东区搬走）

the East End（伦敦东区），在历史上曾是一个拥挤的贫民区。街道狭窄、房屋稠密，多为十九世纪中期建筑。第二次世界大战中，大部分遭受轰炸破坏，后重建。是伦敦传统工业区有服装、制鞋、家具、印刷、卷烟、食品等工业。

划线部分含有 who 引导的定语从句。意思是：

那些能够负担得起更加现代生活环境的人（从东区）搬了出去，这一地区变成了绝大多数人生活在极度贫困之中...的一个地方。