Plagiarism is using a source's words, ideas, or work without giving it credit. This can give your audience the mistaken idea that you came up with the material on your own.
If you plagiarize, you could get a lower grade, fail an assignment or class, or worse.
It's important to cite other people's work-- even if it's really old, even if you find it on the internet, even if the person who wrote it is dead, and even if you re-worded everything.
Make sure you keep track of your sources as you go! Otherwise you may forget where your notes came from.