Plagiarism is when you take someone else's work and claim it as your own or fail to cite the original author. Plagiarism also includes taking your own work from past classes and using it in your current class. It even includes copying another classmates work.
You can avoid plagiarism by properly citing when you are using someone else's words or ideas.
Plagiasm is not allowed in school, college, and beyond. So it is not a good idea to plagiarise someone else's or your own work.