Plagiarism is presenting work or ideas from another source as your own, with or without consent of the original author, by incorporating it into your work without full acknowledgement.
Plagiarism is a big deal. We want to make sure we are not stealing others work and taking credit for it. How would you feel if you worked really hard on a project and someone else took the credit? Probably not very good. So, to avoid putting that feeling on others, let's do our own work and site the sources we get our information from.