a string metric for measuring the difference between two sequences. Informally, the Levenshtein distance between two words is the minimum number of single-character edits (i.e. insertions, deletions or substitutions) required to change one word into the other.
Source: Wikipedia: Levenshtein Distance
Why does this matter?
Levenshtein distance is a form of approximate string matching. In real life application, Levenshtein distance can be used in spell checking, correction in OCR system (Optical Character Recognition), record linkage, auto suggest words in smart phone and many more.