Isilber17 is correct I shall try to explain.
There are 6 letters and they are all different.
I will assume that the letters cannot be repeated - I think that is the intended meaning of the question.
You choose the fist letter there are 6 possible outcomes.
now you choose the next letter, there are 5 possible ourcomes because one has already been chosen.
so the number of ways 2 letters can be chosen is 6*5
now you choose the third letter. there are 4 possible letters available to choose from.
So the number of ways you can choose 3 letters is 6*5*4
etc
the number of ways you can choose all 6 letters where the order is very important is 6*5*4*3*2*1 = 6!
Does that help?