Interpolation is mathmetically filling in missing points from
sampled data. For
example lets say you go out and measure the position of the moon every 15
minutes. You don't actualy know where the moon was in between those
measurements. But you can guess. The simplest interpolation would be
a straight
line between observed samples. For example if you observe that the
moon is at 20
degrees at 2:30 and 22 at 2:45, then you can guess that it was at 21
degrees at
2:37:30. (I am making the measurements up, but you get the point.)
When you blow up a digital picture one of your options is to make
all the pixels
bigger. This generally looks hideous. Another option is to move the pixels you
have to where they would be in a larger picture, and then to fill in all the
white space by interpolating the values that are in between your
original pixels.
I don't know what kind of model photo shop or similar would use, but I imagine
it would be more sophisticated then a straight line.