I get this question all the time, and it's a fair one. The phrase "single origin" gets thrown around a lot by those deep in the coffee lifestyle, and it's not super understandable to someone who's just getting started.
Once you break it down though, it makes sense: a single-origin coffee comes from one origin, or source country (Brazil, Kenya, etc.). Usually, more specifically, it comes from one localized region, or even one coffee farm. When coffee nerds say "single origin," they often mean a region, like Yirgacheffe, or an individual producer, like Hacienda La Esmeralda.
What's more complicated is why this matters. I think it's helpful to first understand that until the 1990s or so, much of the coffee we consumed in the U.S. was in the form of blends--coffee purchased from around the world based on price and availability, tossed together and roasted until it all tasted the same. (This is why many people think "coffee" is a specific taste all on it's own, while coffee nerds are walking around talking about how coffee tastes like blueberries and such.)
Around the same time as the rise of Starbucks, coffee professionals started looking at beans more like wine experts look at grapes, considering the spectrum of varieties and terroir, and how it affected the final cup. Some even started forming relationships with coffee farmers at origin, in order to see specifically where their coffee was coming from. The environmental and soil conditions in which a coffee grows have a huge influence on its flavors--Kenyan coffees are known for being "jammy," Brazilian coffees are known for chocolate notes, and Ethiopian coffees are known for their full, fruity flavors.
If you're still thinking I'm crazy (Is this you? "But coffee tastes like coffee...?"), you're not alone. It takes an extremely special cup to make you go, "OH, that does NOT taste like coffee!" For me, it was Ethiopian Ardi (pictured above) from the Sidama-Guji zone. It's a natural coffee, meaning it's dried in its cherry, so it soaks up a lot of fruit flavors. By isolating a coffee from one particular area, the distinctive flavors of that region can come through more clearly. Ardi tastes like raspberries and blueberries, and sometimes grape or cherry.
If you're still thinking I'm crazy (tough crowd, tough crowd), then consider this: single-origin coffee matters because it's traceable, it's more direct, and sometimes it benefits coffee workers and small producers. In a food supply chain that encircles the globe in tangled, confusing networks, even the smallest effort at transparency matters. Knowing the name of the mill where your coffee was processed, that matters. Buying a direct-trade coffee that passes premiums onto the farmers, that also matters.
So while the coffee nerd talk might feel alienating sometimes, don't (always) write it off as snobbery or needlessly expensive beans. And the first time you order a single-origin coffee, try it without milk or sugar. Swirl it around in your mouth and really pay attention. You might taste something you never tasted before!