Luhn Algorithm Calculator
The Luhn algorithm is used to validate various identification numbers, such as the numbers on the front of credit cards, social security numbers or the IMEI of a phone.
The last number of a Luhn sequence is the control digit. If you enter a digit incorrectly, therefore the algorithm will calculate a different checksum. So, the Luhn algorithm is well suited for checking numbers entered by people. In case they have a typo in their sequence, then the last digit will fail the validation.
If you want to see the validation method, take a look at this answer.