Color profile / ICC profile
A small file that tells software what a device's color numbers actually mean, so an image can be translated between screen, press and paper.
See it
What it is
A device's 'red' is just three numbers until something says what they mean. An ICC profile is a small file that maps one device's numbers to an absolute reference (CIELAB), so color management can translate source profile to reference to destination profile. On the screen side you meet sRGB and Adobe RGB; on the press side, profiles like GRACoL, FOGRA39 or US Web Coated (SWOP). A good print shop will hand you theirs, and that is the one that wins.
Reach for it the moment a job crosses devices or a printer asks 'what profile is this'. Two knobs matter. The assigned profile says what the existing numbers mean. The rendering intent decides how to handle colors the destination cannot reach: perceptual squeezes everything smoothly (good for photos), relative colorimetric keeps in-gamut colors exact and clips the rest (the usual print default, with black point compensation on).
Gotcha: assign and convert are opposites, and mixing them up is the classic disaster. Assign changes the meaning, so the appearance shifts. Convert changes the numbers to preserve the appearance. Worse is an untagged file: with no embedded profile every downstream tool guesses, usually sRGB, and your Adobe RGB photos go flat. Embed profiles on export, and never convert the same file twice.
Ask AI for it
Set up color management for this print job end to end. Tell me which ICC profile to work in and which to convert to for [output: coated offset / uncoated offset / digital press], which rendering intent to use and whether to turn on black point compensation, whether to embed or leave out the profile on export, and the exact steps in [tool] to check that no placed image is untagged and nothing has been converted twice. Finish with the PDF/X export settings that carry the right output intent.