CMYK
CMYK is the four-ink color model — Cyan, Magenta, Yellow, and Key (black) — that professional printers use to reproduce color on paper. It has a smaller color range than the RGB your monitor uses, so bright neon greens and electric blues on screen tend to come out duller in print.
Why it matters
You meet CMYK the moment a print shop asks for a "CMYK PDF" or you notice your vibrant on-screen colors look muddy in the proof. Converting early lets you adjust before it's expensive.
Example
A vivid pure-blue button on screen (RGB 0, 0, 255) might convert to CMYK (88, 77, 0, 0) and print as a darker, slightly purple navy. Soft-proofing in CMYK shows you what you'll actually get.