Glossary
CMYK
CMYK (cyan, magenta, yellow, key/black) is the four-ink colour model printing uses. Screens use RGB, which can display colours no ink can reproduce, which is why a print can look duller than the file did on your monitor.
The colours that shift most are bright, saturated ones: vivid blues, neon greens, and intense oranges commonly print noticeably flatter than they appear on screen.
You do not need to master colour management to avoid the worst of it. Simply knowing that a screen-perfect neon will not survive the transition is usually enough to choose a design that prints well in the first place.