(English is not my first language, but I think you will get the picture...
)
Reducing pixels and reducing resolution can be the same thing. 2000 x 3008 could be the "pixel size" of an image. Resolution tells you how these pixels are displayed/placed. Were you to show this image in all its actual pixels on a screen with 72 ppi it would be 27,8 x 41,8 inches in size. If you change resolution, but keep the "pixel size" the original amount of pixels the image consists of, the parameter that is changed is the size in inches. At res. 300 the size will be 6,7 x 10 inches, and still 2000 x 3008 pixels.
For printing (real printing) an image and get good quality it should be res. 300. Then your total amount of pixels constrain you in how large print you can get, in inches. Increasing the resulution in the computer without porportional reduction in size will not give an image of higher resolution in reality. The information in the exsisting pixels will just be spread out on some more pixels. But likewise, an image isn't any better, regardless of it's massive amount of pixels, than the information saved in these pixels. A digital camera can have all the pixels in the world, but poor optics and not good enough techniqe to fill these pixels with useful information.
I don't know if I've written anything you didn't already know and I would gladly be of further assistance if you think I can help you, or anyone else for that matter...