Imagine your bog-standard digital camera or phone camera.
Your digital camera is 12MP, but the screen you view the images on is only 0.5MP.
However, if you were to view the image on your computer then you would have 2MP. If you zoom in you can split it into 6x 2MP seperate images. Each with the same level of detail as the zoomed out image.
Does this make sense? Just because your device can't show the full 12MP (0.5MP on your camera) doesn't mean they are not there, just you can't see them unless you zoom in.
Why is this useful? You have a face you want to recognise in your 4MP picture. You only have a 1MP monitor. You can't see it clearly. If you had a 1MP camera, you can't improve that. But with your 4MP camera, you can zoom in as each 1/4 of an image is 1MP. You zoom in to the quarter where the face is and you can now see it clearly.
That's generally why more MP is better but don't be fooled by more MP is always better. Some lower MP cameras have less noise and better image + better night-vision. I have 1MP, 2MP, 3MP and 4MP. For me 2MP is the sweet-spot and I use 4MP for the porch as it has a wide-angle lens (2.8mm).