1、名词性从句（主语从句、宾语从句.表语从句和同位语从句）有4种类型：
that 从句：陈述已知事实wh- 从句：陈述探明具体情况的事实if/whether 从句：陈述供选择的事实what 型从句：所陈述“所…的”事实
这里的两个句子是 what 引导的名词从句.
2、 what 型也称关系代词性从句, 等于“先行词 + 关系代词” 的合成,相当于上面所说的汉语“所…的”.如：
(What happenedafter that) was interesting. 在那以后“所发生的事情”很有趣.【主语从句】He told me (whathe had learned). 他告诉了我他“所了解到事情”.【宾语从句】This is (what I know).这是“我所知道的”.【表语从句】
3、问题中提到的两个句子都是同位语从句
1）The world's population has grown six times (what it was in 1800). The figure is nowapproaching 6.5 billion.
what it was in1800 是用来进一步说明 six times（六倍）,即“这个倍数是1800年曾经有过人口的倍数.因此,此处也可以改写为The world's population has grown six times (one that it was in 1800).改写后的结构如下：
主语：The world's population谓语动词：has grown程度状语：six times同位语：one （指代the times）定语从句：that it was in 1800”（其中 it 指代 the world’s population,关系代词 that 是表语,指代 one,即 the times.也就是说 that it was in 1800 = the population was the times in 1800）
2、The price ofthe meat is double/twice (what it was last year).
同理,可以改写为 The price of the meat is double/twice (one that it was last year). 【it 指代 the price】