Interpolation
When an image is made larger, the existing pixels cannot simply be stretched, as you might stretch a rubber band. Instead, new pixels must be created. In order to "guess" what these pixels should look like, the software that enlarges the image uses interpolation. The software first spreads out the existing pixels into the new image size, leaving many gaps and spaces. Then, it examines the existing data (the original pixels) and then creates a function that describes the missing data (the new pixels) to create a new data set (the enlarged image). Sometimes, the program does a poor job, and the resulting image is blurry or wrong in places. These are called interpolation errors - the software did not correctly guess what the data should be. But, overall, the image will look similar. This is an example of the risks and benefits of interpolation - the user can see new data, but that data may not be perfect.