An opinion is what someone believes is true but has not been proved.
这个句子是解释opinion的词义，即给opinion下定义。what someone believes is true but has not been proved为名词性关系分句作表语。what = something that, 句子可以改写为：
An opinion is something that someone believes is true but has not been proved.
这样的话，黑体部分为嵌入式定语从句，关系代词在believes 的宾语从句中作主语，is ture but has not been proved为that的并列谓语。所以，原句中，what从句为嵌入式名词性关系分句，what在believes的宾语从句中作主语。网友的理解是正确的。句子可以翻译为：看法指一个人认为是正确的但又没有被证实的观点。
