Nonetheless

It means, no less important

To understand it, let's put it in a sentence and translate it literally and generally: التعليم لا يقل أهمية عن الصناعة. - التعليم: the education - لا: No/doesn't - يقل: decrease - أهمية: importance - عن: than - الصناعة: industry - so literally, it means: The education doesn't decrease (in) importance than the industry. - the accurate meaning in English: Education has no less importance than industry. - So, you use it to show how something is important in a comparison to another thing

It means "it's as important as sth. For example: The scarcity of water issue is as important as global warming problem. مشكلة ندرة المياه لا تقل اهمية من مشكلة الاحتباس الحراري . It literally means It's importance isn't less than ... لا a word for negation يقل to diminish. In this context it means less اهمية importance.