A real plagiarism check compares your text against an index of published work. This tool does not do that, and no browser tool honestly can. What it does instead is score the structural tells that correlate with copied writing: heavy quotation without citations, six-word runs that repeat, stock phrasing, and a narrow vocabulary that suggests a source was reworded rather than rewritten.
Quotation is not plagiarism — unattributed quotation is. If the quoted share is high but every quote carries a source, you have a style problem rather than an integrity problem: paraphrase more and quote only the lines whose exact wording matters.
Everything runs locally in your browser, so unpublished coursework, client drafts and manuscripts are never uploaded anywhere.