The Levenshtein distance is a number that tells you how different two strings are. The higher the number, the more different the two strings are.
For example, the Levenshtein distance between "kitten" and "sitting" is 3 since, at a minimum, 3 edits are required to change one into the other.
kitten -> sitten (substitution of "s" for "k")
sitten -> sittin (substitution of "i" for "e")
sittin -> sitting (insertion of "g" at the end).
An "edit" is defined by either an insertion of a character, a deletion of a character, or a replacement of a character.