Kurt Lewin's core idea was to shift psychology's focus from what he deemed an "Aristotelian" to a "Galileian" approach. That is, he suggested that psychology needed to follow physics and have less focus on the idea of static, unchanging concepts and more focus on psychological dynamics.

One of the major cores in Lewin's work is the idea of a "life space." A "life space" is the combination of all factors that influence a person's behavior at a given moment in time. Therefore, a life space may include instantaneous thought, memory, drives and motives, personality, as well as the situation and external environmental factors. This idea is prominent today with the idea of the person-situation interaction.

Field Theory is essentially an attempt to provide a somewhat empirical approach to capture a person's life space analytically. A person's "psychological field" emerges dynamically in relation to the individual's life space. Lewin suggested that the fundamental ways to get at a person's "psychological field" included a) avoiding rote classification, b) focus on the dynamic properties of events (e.g. how thought changes in the situation), c) a psychological approach instead of a physical approach, d) focus on the entire situation - not on individual components in isolation, e) always consider time, and f) develop a mathematical representation of the psychological field. (These ideas are well summarized in Sahakian's 1975 book on psychology history.)

In sum, Lewin's theory is basically that people are dynamic creatures with dynamic thoughts, emotions, and psychological forces. To understand people from a psychological perspective, Lewin advocates that one considers all possible factors that influence a person's behavior and consider how those factors interact and change in time to influence the person's present state.

Hope this helps!