Least is the superlative form of little and can be used in the following ways:
as a determiner (followed by an uncountable noun): He generally takes the path of least resistance. (after ‘the’): These farming methods cause the least damage to the environment.
as a pronoun (without a following noun): People will choose the product that costs least. (after ‘the’): Abe works the hardest, yet he earns the least. (followed by ‘of’): That’s the least of my problems.
as an adverb (after ‘the’ and before an adjective): I think you are the least suitable person for the job. (followed by an adverb): Production was slowest where time was least efficiently used. (with a verb): She was kind to me when I least expected it. (after ‘the’ with a verb): The money seems to go to the people who need it the least.
as an adjective: At the least sign of trouble, he gets scared and quits.