Passing Customs

I always cringe a little inside when I pass through customs, wary that they'll check my passport a little too closely or decide to root through my luggage, sniffing out contraband like badgers. Not that I have contraband, and not that there is anything too tricky about my passport, but the fear is that I'll be caught breaking a mini-law that I had no idea of breaking. Is this Snickers bar food? Do I declare it on my customs form? What does this section over "goods acquired while abroad" truly mean? Can I leave it blank because I don't feel like filling it out?