For the 1st in the sequence there are 5 numbers from which to choose.
For each of those there are 4 numbers left for the 2nd in the sequence i.e. there are 5X4 possibilities for the first 2 numbers in the sequence.
There are now only 3 numbers left for the 3rd in the sequence for every one of the 5X4 pairs i.e. 5X4X3 possibilities for the first 3 in the sequence.
Continue this and the total number of possible sequences to make from 5 numbers using all of them is 5X4X3X2X1 (=120), which is also called the factorial - this can be written more succinctly as 5!