Spacing of Navigation Folder / Dropdown Menu

Before I start digging into custom CSS to change things, could someone explain to me why on my site the drop down menu pops up and overlaps the main navigation? (I'm new to 7.1 having recently upgraded from 7.0, and I'm trying to fix all the things that broke in the transfer).

firstlutheranonalaska.org

Thanks for the link to the solution @tuanphan, I had no clue. Much appreciated.