When you mention a lot of things and when you come to the last one, sometimes you use this expression (last but not the least), it means that though it is mentioned last, it is as important as the other things mentioned. Because it is mentioned last it should not be thought that it is least important.

Last but not least, I should like to thank the Vice-Chancellor for accommodating us in the hostel.

Please note that the expression is ‘last but not least’. There is no ‘the’ before least.”