PSI (pounds per square inch) is the air pressure your compressor delivers to the brush. Miniature and kit work lives in the 15–30 PSI band: lower for fine detail with thin paint, higher for primers, metallics, and heavier bodies.
A regulator with a gauge — not the compressor’s max rating — is what matters; you tune pressure to the paint in the cup. A useful habit: set pressure while the trigger is pressed (working pressure), since static readings drift several PSI higher.
See it in context in our compressors & air guides, or explore the tools hands-on in The Bench.