As a Build Engineer my job can be kind of tough to explain. I don't work on gameplay, I'm not an artist or a designer...And most people don't really know what the concept of a "build" is if they don't already work with them (especially since even within the field there are so many different kinds of builds and definitions for builds). So when people are like "So...what does a build engineer do exactly?" you kind of end up having to explain to them first what a build is and what it's used for (trying not to get carried away with different kinds of builds) and then explain what you do in the context of that.
I usually start off saying something like: "I work on and manage the team's continuous integration and autotesting systems that produce different kinds of game builds that the rest of the team then works with or that we send to third parties or certification."
Then they're like "What does that mean? What's a build?"
And to try to find the simplest explanation for it I say something like "It is this...package of the game. Kind of like when you install the game on your PC, you are installing a 'build' of it."
And then it turns into a whole discussion...I still haven't found a good way of one-lining it.
How about "I manage bringing together all the work from everyone else on the team and bundling it together so that it works as a single program for the end user rather than random bits and pieces."?