Hand gestures, much like words can have different meanings in different places. It pays to learn a little about the country you will be visiting to avoid confusion and an honest mistake (which just might piss somebody off)!
In the U.S. we shake our head up/ down for yes and sideways for no. However, in some countries this is not the case. Over the summer we visited a few countries in the Balkans. I don’t recall the exact rules but in Albania & Macedonia a head nod actually signaled the opposite of what we’re used to!