When creating a password the following characters are usually available. Enter the values then click on Calculate. Basically it shows how many different possible subsets can be made from the larger set.
What is a combination.
For a password to be difficult to crack it should be chosen randomly from a large set or space of possibilities. If someone rolls a die twice you know it can. The maximum number of password possibilities for the values you entered is returned. N - r where n represents the total number of items and r represents the number of items being chosen at a time.