Plagiarism is taking someone's else's words or work and representing it as your own. It is way more common that you think and sometimes the area is grey and it can be difficult to tell whether it is plagiarism, or you are just writing something that is well known.
If you are borrowing someone else's ideas and you do not cite them, you are plagiarizing. A good rule of thumb is to cite everything. That is, if you think this information is well known, for example you saw it in a textbook or it is used commonly in reviews, then cite the textbook or review.
Plagiarism.org is a fantastic resource for learning more about plagiarism. I'm sure you know that using someone's actual words and not citing them is plagiarism. However, borrowing their ideas is also plagiarism. Learn more about how to prevent this! Note: Borrowing ideas from your mentors, peers, labmates and classmates is also plagiarism.
What if you are reading a manuscript or review and you want to cite something in that paper, but the authors of the paper don't cite where the ideas came from and you can tell that the authors did not contribute those ideas? How you can find where those ideas originally came from? Learn more about how to find the right sources.
How can you avoid plagiarism? Through planning when you write your paper. Learn more about tips and tricks for preventing plagiarism when you write.
How can you check if you have plagiarized? Use an online plagiarism checker. Here are a few: Grammarly, SmallSETools, PlagiarismChecker, QueText, CopyLeaks , etc.