"Print-ready" gets thrown around loosely, but when a print provider — POD platform or local shop — actually uses the term, they mean a specific, checkable set of criteria, and missing any one of them is usually why a file comes back rejected or flagged.
Resolution is the most obvious one and the one most beginners already know to check: the file needs enough pixel density (typically 300 DPI at final print size) that the design won't look soft or blocky once enlarged to the actual product dimensions. This is worth double-checking against your specific intended print size, not just assuming a file is "high-res enough" in the abstract.
Color mode matters more than most sellers realize. Many print providers, especially for screen printing or professional press work, expect files in CMYK rather than RGB, since that's the color model the physical printing process actually uses. An RGB file isn't necessarily rejected outright, but colors can shift unpredictably during conversion if it's not accounted for ahead of time — some providers handle this conversion themselves, others expect you to submit CMYK already.
Transparent background, for anything that needs to sit cleanly on a colored product, is a near-universal requirement — a file with a visible white box where the background should be transparent is one of the most common reasons a file gets flagged and sent back for correction.
Bleed and safe zones come up specifically for full-bleed prints (designs that extend to the very edge of a product) — providers often want a small amount of extra design extending past the visible edge (the "bleed") to account for minor cutting variance, plus a safe zone where important content like text stays clear of the edge entirely. Skipping this is a common reason full-bleed designs get sent back with a request to adjust margins.
Embedded fonts or converted outlines matter if your file includes live, editable text rather than an already-flattened image — a provider working with a layered file needs either the actual font file included or, more commonly, the text converted to outlines/paths so it displays correctly without needing the exact font installed on their end.
File format itself, naturally, needs to match what the specific provider actually accepts — some platforms only take PNG or JPG for upload, others expect a vector format like AI, EPS, or PDF for anything going to screen printing or professional press work. Checking the specific provider's accepted format list before finalizing a file, rather than assuming "high quality" alone is sufficient, avoids the single most avoidable reason files get bounced back at the last step.