“the process of determining the value of a function between two points at which it has prescribed values.” - Dictionary.com

Basically, interpolation is the common sense concept of "The hill behind me is low, the mountain in front of me is high… therefore I must be somewhere in between low and high." However simple this may seem, when you have 3 or more hills at varying distances, it can get complicated. That is why we need specific algorithms such as Microsphere Projection to handle these types of difficultiles.