Basic dictionary definition:
"to steal and pass off (the ideas or words of another) as one's own: use (another's production) without crediting the source"
Plagiarize (and plagiarism) comes from the Latin plagiarius “kidnapper.”
But what does that really look like in practical terms?
The exact consequences will depend on your professor, and they will lay out their policy for handling plagiarism in the course syllabus. Possibilities include:
*These are things which would appear on your transcript, which other schools and possibly future employers will be looking at.