On a screen printing press, every color comes from a separate screen. Registration is the work of aligning those screens so each one prints in the right spot on every shirt. Printers rely on registration marks on the films, pre-registration systems and fine adjustments on the press to get there before the run starts.
Misregistration shows up as gaps where two colors should meet, a white edge peeking out from under a color, or doubled outlines. Separators plan for small movement with trap, a slight overlap between touching colors, and by pulling the underbase in so it hides inside the top colors. Knit fabric stretches and shifts on the platen, so designs with many tightly butted colors are harder to hold than bold, outlined ones.
If your design depends on hairline gaps between colors, ask the printer whether it will hold on your garment. The artwork and separations guide covers trap and choke.