Gestalt Principles are principles/laws of human perception that describe how humans group similar elements, recognize patterns and simplify complex images when we perceive objects. Designers use the principles to organize content on websites and other interfaces so it is aesthetically pleasing and easy to understand.
A «Gestalt» (Origin: German) is perhaps best described as a «form» or «pattern», however this does not give full justice to the word, which has no direct equivalent in English. In the context which Perls placed it into, a gestalt entity is something which is greater than the sum of its parts. In order to understand an event, or something or someone, you must consider the entire situation and the components which go to create that particular form at that point in time.