Void context is where you have an expression (not to abuse Dominus' terminology too much) that doesn't go anywhere. For example, if you had:
in reply to Re: Re: Re: Scalars, Lists, and Arrays
in thread Scalars, Lists, and Arrays
Ignoring the fact that the program does nothing, the second line is a scalar expression (help me out here, if I abuse verbiage) in void context.
open(FILE, $filename) or die "Can't open $filename: $!";
"nothing to see here";
When you abuse map to iterate over a list and don't assign the results to anything, you're using map in void context.
It's generally not useful, but if you have an expression that can spend a lot of time creating a result value of some sort, it can be inefficient to use void context.