For the future, this should be posted in the questions section, not in the notebook section.

Without context, it is very difficult to answer this question. Neither "not least" nor "at least" ever appears on its own; they both must be part of a phrase.

"At least" is primarily used in two ways:

"We were going to go on a picnic today, but it's raining. At least we still have all this delicious food to eat!"

or

"You never participate in class. Pay attention, at least."

The former uses "at least" to say that there is something good to make up for something bad, or to say that it's not all bad. "That food was terrible. At least it was cheap." Etc.

The latter says "If you don't do anything else, do X."

"Not least" is used to emphasize how important an aspect of something is.

"I love my apartment, not least because it is quiet."

"That's a terrible movie, not least because of the bad acting."

Sometimes "not least" is written as "not least of all". These mean the same thing.

Hope this helps!