Wikipedia is your friend if you don't understand words such as display resolution, easy as that.
Pixel doesn't directly correlate to size on the screen, since obviously it depends on how large one pixel is on the device. Maybe you can understand this sentence at least.
If you code in 16:9 in a smaller resolution and scale up to a bigger 16:9 resolution, then the size of your objects will be the same as if you had programmed it for the bigger 16:9 resolution. Only difference will be that it won't be as sharp / image quality will be worse, because you have less information available for each pixel. (talking about raster graphics here)