Let try to make it easy
Let say you have a picture the size ise 4pw wide and 6px high. That the amount of detail you have for you image not much at all. Now let print it. How big will it print. That depends on the Print resolution. So in image size we will uncheck resample for we want to print our image not a scaled version. Print the details we have. So set the dpi 1 make is simple 1px will print 1"x1" in size so our image will print 4"x6" we will see all our 24pixels with ease a ver soft boxy image. Normal print resolution line 300 DPI I image will be printed them 4/300"x6/300" or 1/75"x1/50" need a loop to see it, To print an image that is 1"x1" with 1/300x1/300size pixels you need a lot of detail 300pxx300px=90,000 for a 4"x6" print 1200x1800 px 2.16MP.
If you resample our original 24px up to 2.16MP and print at 300dpi you will get the same boxy picture we start with foe we have very little details interpolation is not magic Any time you resample interpolate change the number of pixel you have up or down in number you loose some image quality. Reducing the number of pixels requires you to discard details you have for your image. Increasing the number requires you to make up details you do not have for your image. So the best pixels you have for you images are the pixels your camera gave you. Details captured with a good lens in focus a high quality large sensor many MP to capture fine details, good lighting and firmware. You want to preserve these pixels you do not want to resample them without a good reason.