As froggy said, "native resolution", at least as applied to LCD displays, is the number of physical pixels your monitor displays.
"Upscaling" is a bit more complicated. Running a game rendered at 1080p on a 1440p or 4K display won't get you a game displayed in the native resolution of the screen, it will get you a game rendered in 1080p displayed on a screen at its native resolution. In other words it will still look like a 1080p game (for the most part). But that isn't exactly what upscaling means in today's vernacular. Upscaling refers to rendering a game at a higher pixel density than the display is capable of and then scaling it back down to that native resolution. This can result in a sharper, cleaner image, kind of getting the benefits of the higher resolution without having to buy a more expensive monitor. It won't substitute for running a game at 4K on a 4K display, for example, but it can very much improve the image in some games.
Many games today have the ability to adjust upscaling. Note though that it takes more GPU horsepower to do this so frame rates may become a problem - but if your favorite game offers the ability it's definitely worth checking out!