What are Gestalt principles and how do they apply to interface design?

Gestalt psychology describes how people see patterns: proximity (things near each other group), similarity (same color or shape), continuity (smooth lines), closure (missing pieces still read as a whole), and figure–ground (what pops forward vs the background). In UI, you use spacing and alignment to form groups, not only boxes; you use a consistent color for 'selected' to show a set; you avoid lines that run past where they are needed, which can trick users into false grouping. In interviews, you show you think about preattentive processing: users decide where to look in milliseconds, before reading.

[ Item A  Item B ]  [ Item C  Item D ]  # two groups by gap, not only borders

Start simple: try this concept in a tiny project before moving to advanced tools.