Gestalt is actually a psychology term and it hypothesizes that people tend to organize visual elements into groups when certain principles are applied. The word literally translated into "unified whole" and this theory was developed by several German Psychologists in the 1920s.
For example, if you see squares that have the same color and size placed next to each other, you perceive them as having a relationship rather than as two separate elements.
Basically, according to Gestalt psychologists, the whole is different from the sum of its parts. Therefore, the whole is more than its individual elements.