Today we are going to be rather technical. You may not know what a citation is, but be patient and all will be explained.
A hot topic these days is plagiarism (another difficult word). This means using materials that are not your own. Someone who has to write an essay or do a project looks on the internet, finds some good material and puts it into their essay as if they had written it themselves.
This is lazy and dishonest. Also, it is foolish as usually it is very easy to spot plagiarism. Suddenly the style changes, there are no mistakes and then after a few lines one returns to the old style. It is obvious you have put a piece of someone else's writing in your text.
The answer is simple. Be honest and tell people where you found the idea or words. Giving someone a reference to a book, newspaper or website is called citation.
University students need to do it all the time, and secondary school students should get used to it for their larger pieces of work and projects.
One of the most widely used systems is APA. Using this style, each time you refer to a source you give a reference briefly in brackets straight after it.