Plagiarism is the act of copying someone else's work. It is a very important issue and one your teachers take extremely seriously. All students should understand that teachers are skilled professionals. They can tell when you are copying work. There are also online programs that scan submitted papers and identify any passages that are identical to each other or found elsewhere on the web. Should you try to plagiarize, you're not just trying to cheat your teacher, you're trying to beat technology too.
Plagiarism is theft. You are stealing another person's work. Not only is this dishonest, but it also risks opening you up to severe academic consequence (see next point).
Also, there is no educational benefit to plagiarizing something. You have demonstrated the ability to copy and paste. You have not thought about or understood the material. This will leave you further behind the other students in your class and less likely to keep up with future work.
The consequences for plagiarism can be severe. Students caught plagiarizing can expect to receive a zero for the assignment in question. Your parents or guardians will also be contacted and informed of what has happened.
Be aware that if you go on to college, plagiarism can be grounds for permanent expulsion. This is why it is important to get into good habits as soon as possible.
Students sometimes express concern about accidental plagiarism. Please understand that your writing style is as unique as a snowflake or a fingerprint. As long as you don't copy something, you have nothing to worry about.
Helpful hint: Quoting something is not the same as plagiarizing it. Just remember to use quotation marks and references to show the words are not your own.