it means "don't think……is important"or"look down on……” 用中文解释：认为……不重要 hope my answer can help you!

“当”的意思是“作为，当做”，“回”是量词，表示“事情、动作的次数”。直译“不把…当回事”就是说“不把…作为一件事情来看”，意思表示不把某人某物看得很重要，或者是对某事态度不认真，无所谓。

it means that you don't care about it. because you think it is inferior or unimportant or you regard them with disdain.

sth is not important,you can miss it,in another word ,you don't need put it in your hear

well,it might have various meanings.it depends on different situations when you say this. in general,it have 2 meanings. 1.someone doesn't care about something. 2.someone doesn't take something seriously. for example: 1.他不把我当一回事 he doesn't care me at all. 2.你不要把他的话当回事 you needn't take what he said seriously.