If you've heard the term "single origin," it was probably in reference to coffee. Single origin coffee and a single origin chocolate are basically the same idea, with the origin of their key ingredients (coffee bean and cacao beans, respectively), being from a single place.
“Single plantation” or “single estate” refer to cacao from a specific property, meaning that the cacao was sourced from a smaller area. This can— but doesn't always— mean that the cacao's flavor is more distinct due to specific varietals, environmental conditions, and a shorter distance to travel before starting fermentation. Therefore, single origin chocolate is made with cacao from one origin, or place. But this is where the generality of the term "origin" becomes a problem.
Historically, one's origin is a country, and then a city or even a specific neighborhood. Due to the nature of how cacao is processed in preparation for making chocolate, very little of the world's cacao can be traced back to a region as small as a neighborhood. There are some very small cacao farms in the world, and some of them are selling their cacao to chocolate makers. But this is not the norm. Most chocolate makers use countries as a frame of reference for defining “single origin.”
A chocolate maker using various single origin cacaos will often present their creations as their “Nicaragua bar” or their “Vietnam bar,” and so on. This is problematic because it generalizes the flavor of that specific cacao & harvest as the flavor of an entire country. It also makes it easier for large chocolate manufacturers to buy the cheapest cacao they can find from various parts of a country, and blend it in large batches into a single origin bar. It’s still from one country of origin, after all, and single origin chocolate are generally sold at a premium.
The term “single origin” says nothing of the quality of beans or sugar, and does not guarantee that farmers were paid an equitable price. While a chocolate made with cacao from a single estate is likely directly purchased from a farmer, it’s not guaranteed. That’s why it’s important to look for traceability from chocolate makers.
If you’re going to pay a higher price, it should be for an overall better product— better for the planet, better for farmers, and better for your taste buds.