Plagiarism is "the unauthorized use or close imitation of the language and thoughts of another author and the representation of them as one's own original work" (from Random House Unabridged Dictionary, 2nd. Ed. (1993).
Your professors are busy with discovering and creating new knowledge in their subject areas. They do this by becoming knowledgeable in their subject and undertake focused studies or research. Their results contribute new information for their subject's body of knowledge and is shared with their academic communities.
Transparency & Attribution
However, for their results and discoveries to be considered trustworthy, your professors must show the sources of information they used and how they did their research, known as attribution. This information allows transparency in their work so that interested readers can see where your professor got his or her ideas, and also how they did their research so it can be repeated to see if they will get the same results.
Academic Honesty & Ethical Behavior
The problem with plagiarism is a person does not receive credit or recognition for his or her ideas. Do you want someone to take an idea you had and claim it is theirs? What if you spent hours working on a paper only for someone to copy it and earn the same grade as you? You were the one who did the hard work, so was that fair? No, it wasn't!
The bigger deal about plagiarism is that it is unethical! Ethical behavior is a foundation of academic work, and it is based on norms of mutual trust, personal integrity, honesty, and individual responsibility. Behaving unethically affects the educational experience that aims to be fair and create a level playing field for its students. Plagiarism is like stealing and cheating!
Assignment Requirements & Communication
In college assignments, most professors will require you to document your sources of information so you can identify (and give credit to) the person who had the original idea. It also helps show how you got your ideas and came to your own conclusions. Showing your work also prepares you to learn the conventions of research and communication in academic, professional settings.