Plagiarism is essentially a form of theft, of stealing someone else's ideas and claiming them to be your own. There are several reasons why plagiarism is bad. The most important is that you are not only stealing ideas from other people but you are depriving yourself of the benefits of your education. For example, if you cut and paste from a summary or online answer, you have not developed the skill of reading long books and finding salient information, nor have you improved your own writing, research and critical thinking skills. Thus when those skills are required on the job, later in your life, you will be very badly prepared and do badly, leading to being stuck in low-paying entry level positions or unable to find jobs.
To avoid plagiarism, the first step is to do the assigned readings and think about them critically; if you know the material and have something to say about it, you will not be tempted to plagiarize. Next, be careful to cite the sources for ideas as well as direct quotations.