I would like to add something to the answer given by Arunav Khare. He has given detailed explanation and I would like to tell a shortcut here.. If F1 and F2 are two functions, then F1.F2 will contain the minterms present in both F1 and F2.
In our example F1= (1,2) and F2 (0,3). Since there is no minterm common between F1 and F2. So F1.F2 = ∅ and F1.F1 = (1,2)