Keyword stuffing is the practice of repeating target keywords on a resume in an effort to game the ATS keyword-match score. Classic examples: listing "SEO SEO SEO specialist search engine optimization SEO" in a skills section, or burying a white-text block of keywords at the bottom of the document.
Two reasons to avoid it. First, modern ATS use term-frequency weighting and semantic matching — naive repetition doesn't compound score the way it used to. Second, the resume still has to be read by a human recruiter, and stuffing reads as desperate or spammy.
The right approach: identify the 5-10 terms that appear most frequently and prominently in the job description, then use each once or twice in natural context — the summary, the skills section, and specific achievement bullets. That pattern maximizes both ATS score and human readability.