Big O notation allows to specify the complexity of an algorithm in a simple formula, by dismissing lower-order variables and constant factors.

For example, one might say that a sorting algorithm has O(n * lg(n)) complexity, where n is the number of items to sort.
Big O notation is used in Computer Science to describe the performance or complexity of an algorithm. Big O specifically describes the worst-case scenario, and can be used to describe the execution time required or the space used (e.g. in memory or on disk) by an algorithm.