A two-part idiom (歇後語); the second part is 點極都唔明. 牛皮燈籠 literally means “a lantern mounted with hide”. So this lantern emits no light no matter how it is kindled, i.e. 點極都唔明 . The pun lies in the 2 words 點 and 明, which can also mean “to teach” and “to understand” respectively. Therefore, apart from the apparent meaning of “no light no matter how it is kindled,” 牛皮燈籠 also contains a hidden meaning of “not bright no matter how one is taught.”