The reason why we have used IBUFDS is that the board we have chosen requires a differential clock.
Each board will have its own specific pin LOCs and the System Clock frequencies that it can support.
This board can support 300Mhz and 125Mhz, so we are using 125Mhz and the Pin LOC which corresponds to it. The output is connected to an LED which is LOC'ed at AG14. The last 2 statements are used to specify the IOSTANDARD for the ports.