That means despite being mentioned after everyone else it is an important part; for example : " I would like to thank my publisher, my editor and , last but not least, my husband. " Hope this helps :-)

"...the LAST thing, BUT NOT the LEAST important thing..." It's also used often when introducing several people: "...and last but not least, this is Naomi." This nicely says that the last person to be introduced is not therefore the least important.

last but not least 最后一个，但不是不重要的 (Collins (En-Ch))

Basically what you are trying to say with that sentence is that although you are saying it in last place, it is as important as the other things you said before it.

You say "last but not least" when you want to list something without a specific order, like a list of people for example, and you reach the last element of the list: "john, marie, frank.. and last but not least richard".