While you are reading this text on your computer screen, your eyes and brain are carrying out optical character recognition without you even noticing! Your eyes are identifying the patterns of black and white dots that make up the characters (numbers, letter etc.) typed on the screen and your brain is using those to puzzle out what is written (either by reading individual characters or by scanning entire words at once).
This is what computers do with the OCR technology, but this is actually really hard for them. Computers can't read like your eyes can! So in order for them to read something, it must be presented as an image generated with an optical scanner or a digital camera. The picture created this way is just a graphic file and there's no difference between this picture and a photograph of a person or a landscape or any other graphic, it's just a pattern of pixels. The computers can't read the words on the page just like that, like we do, because for them it's just a picture of the page rather than the text itself. So basically what OCR does is to turn a picture of text into a text itself, it looks at each line of the picture and attempts to determine if the black and white dots represent a particular character. So, instead of your computer recognizing a picture as just that OCR software is used to take that picture and identify the real text contained within.