In order to enter input, you must provide an expression with combination and permutation statements. For instance, an acceptable input would be 55C17 - 16C5 * 11P4. This would multiply 16 choose 5 and 11 permute 4 before subtracting it all from 55 choose 17
Combinations and permutations are essentially functions that allow you to determine how many ways something may be organized. A simple example of a permutation is the number of ways you could choose a President, Vice President, and Secretary of an 8 person club. This would be 8 permute 3, written commonly and in this calculator as 8P3.
Conversely, an example of a combination could be the number of ways you could choose a 3 person leadership team in the same club where each leadership position is not distinct. Because of this, the permutation example would be greater by a scale factor of 3!.
If you would like information, we would strongly recommend watching the below Khan Academy Video.