Plagiarism is copying another person's ideas, words or writing and pretending that they are one's own work. It can involve violating copyright laws. College students who are caught plagiarizing can be kicked out of school, and writers who plagiarize will often be taken less seriously.
When writing papers, many students practice plagiarism without knowing it by using other people's ideas without citing them (saying where they got them). Reading another article or book and taking an idea from it and putting it into one's own words is not plagiarism if the writer of the paper says where they got the idea.