There are several significant details that I would like to tell you in advance中的that I would like to tell you in advance
是定语从句，引导词that作tell的直接宾语，tell sb sth。定语从句的引导词在定语从句中充当一定的成分（作主语同位语从句和定语从句、宾语、定语、状语），而同位语从句的引导词在从句中不充当成分，其前面的词往往是idea/opinion/suggestion/advice/ view/news等，同位语从句所表示的是观点、建议、新闻的具体内容，从句与前面的idea等是同位关系，不存在修饰与被修饰、限制与被限制的关系。再看
Do you agree with the idea that/which I came up with yesterday?（定语从句）
Do you agree with the idea that we should finish the work in two days?（同位语从句）
希望我的回答可以帮到你！
