The star refers to items which are intended for the advanced learner.

标有星号的项目是给高阶学习者的。

《牛津词典》

The advanced learner can use her skills to assist the lower-level learner. The lower-level learner is able to learn from the more advanced learner.

高水平学习者可利用其能力帮助低水平学习者，而低水平学习者则能从高水平学习者处学到很多。

youdao

Collins. Cobuild. Advanced. Learner 's. English. Dictionary. Fifth.

[柯林斯高级英语学习词典(第五版)]。

youdao