What does this notation mean: ?

Thisis known as a "permutation"   It means we are choosing 2 things from a set of "n" things.....and the order of these things matter

For instance, suppose we have the set  {1, 2, 3}

So, the possible sets are

{1, 2} {2,1} {1,3} (3,1} {2,3} and {3,2}

This is in contrast to a "combination" where the order within the setsi is irrelevant....so {1,2} and  {2,1} are the same things in a combination.