Dots Per Inch. Whether it be on paper or on a computer screen, the idea is the same: the paper, or screen, is broken into a grid, like graph paper. Each square is one dot/pixel. These dots/pixels then are used to build an image. In most cases, the pixels are too small to see and allow the image to blend together smoothly. All computer screens display images this way, so everything you see on a computer screen is broken down into these small pixel or dots.
A good visual representation of this is the painting, "A Sunday Afternoon on the Island of La Grande Jatte" by Georges Seurat. Seurat created this painting by applying the paint in little dots, much like pixels. This painting could be thought of as a low resolution image blown up to the point where you can see each pixel. Much like in the two examples below. The first is normal resolution, the second is the same image, but blown up so we can see the individual pixels.
But, if you take those same 90000 pixels and stretch it out over a three inch by three inch square, then the resolution is lowered to 100 DPI. That is still not bad, but if you look closely, most people will be able to see the individual pixels and so the image will not be as sharp.
Most computer screens are somewhere between 72 and 90 DPI. A good quality print requires somewhere in the range of 150 to 300 DPI.
Note that a good quality print out has to be about double the resolution of a computer screen. Why then do the lower resolution images of our computer screens look so good? Because we are used to seeing it that way. Since we started watching TV, our eyes have grown accustomed to seeing images on screen at a low resolution. Standard definition TV is in fact, very low in resolution with a total of 480 vertical lines of resolution.