A plagiarism checker is a tool that is used to detect instances of plagiarism in a document or text. Plagiarism is the act of using someone else's work without giving them proper credit or permission. This can include copying and pasting text from a source, paraphrasing someone else's work, or using someone else's ideas without giving them credit.

A plagiarism checker works by comparing the text in a document to a database of existing texts to find similarities. The tool checks for matches between the submitted text and other online sources such as websites, books, articles, and other academic papers. The software will flag any instances of matching text, highlighting them for review.